diff --git a/include/swift/AST/ModuleDependencies.h b/include/swift/AST/ModuleDependencies.h
index 1b47cdafea1..86e8071aad3 100644
--- a/include/swift/AST/ModuleDependencies.h
+++ b/include/swift/AST/ModuleDependencies.h
@@ -1097,13 +1097,8 @@ public:
     return SwiftDependencyTracker(CAS, Mapper.get(), CI);
   }
 
-  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> getClangScanningFS() const {
-    if (CAS)
-      return llvm::cas::createCASProvidingFileSystem(
-          CAS, llvm::vfs::createPhysicalFileSystem());
-
-    return llvm::vfs::createPhysicalFileSystem();
-  }
+  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+  getClangScanningFS(ASTContext &ctx) const;
 
   bool hasPathMapping() const {
     return Mapper && !Mapper->getMappings().empty();
diff --git a/include/swift/ClangImporter/ClangImporter.h b/include/swift/ClangImporter/ClangImporter.h
index 0862c3c94df..74514879e7b 100644
--- a/include/swift/ClangImporter/ClangImporter.h
+++ b/include/swift/ClangImporter/ClangImporter.h
@@ -873,10 +873,19 @@ struct ClangInvocationFileMapping {
 /// `suppressDiagnostic` prevents us from emitting warning messages when we
 /// are unable to find headers.
 ClangInvocationFileMapping getClangInvocationFileMapping(
-    ASTContext &ctx,
+    const ASTContext &ctx,
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs = nullptr,
     bool suppressDiagnostic = false);
 
+/// Apply the given file mapping to the specified 'fileSystem', used
+/// primarily to inject modulemaps on platforms with non-modularized
+/// platform libraries.
+ClangInvocationFileMapping applyClangInvocationMapping(
+    const ASTContext &ctx,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &fileSystem,
+    bool suppressDiagnostics = false);
+
 /// Information used to compute the access level of inherited C++ members.
 class ClangInheritanceInfo {
   /// The cumulative inheritance access specifier, that is used to compute the
diff --git a/lib/AST/ModuleDependencies.cpp b/lib/AST/ModuleDependencies.cpp
index 149a85e514e..dbd5e44c65f 100644
--- a/lib/AST/ModuleDependencies.cpp
+++ b/lib/AST/ModuleDependencies.cpp
@@ -499,6 +499,18 @@ SwiftDependencyScanningService::SwiftDependencyScanningService() {
   SharedFilesystemCache.emplace();
 }
 
+llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+SwiftDependencyScanningService::getClangScanningFS(ASTContext &ctx) const {
+  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseFileSystem =
+      llvm::vfs::createPhysicalFileSystem();
+  ClangInvocationFileMapping fileMapping =
+    applyClangInvocationMapping(ctx, nullptr, baseFileSystem, false);
+
+  if (CAS)
+    return llvm::cas::createCASProvidingFileSystem(CAS, baseFileSystem);
+  return baseFileSystem;
+}
+
 bool
 swift::dependencies::checkImportNotTautological(const ImportPath::Module modulePath,
                                                 const SourceLoc importLoc,
diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index 04616e20531..05f382fef9b 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -1300,49 +1300,11 @@ ClangImporter::create(ASTContext &ctx,
       ctx.SourceMgr.getFileSystem();
 
   ClangInvocationFileMapping fileMapping =
-    getClangInvocationFileMapping(ctx, nullptr, ignoreFileMapping);
+    applyClangInvocationMapping(ctx, nullptr, VFS, ignoreFileMapping);
 
   importer->requiresBuiltinHeadersInSystemModules =
       fileMapping.requiresBuiltinHeadersInSystemModules;
 
-  // Avoid creating indirect file system when using include tree.
-  if (!ctx.CASOpts.HasImmutableFileSystem) {
-    // Wrap Swift's FS to allow Clang to override the working directory
-    VFS = llvm::vfs::RedirectingFileSystem::create(
-        fileMapping.redirectedFiles, true, *ctx.SourceMgr.getFileSystem());
-    if (importerOpts.DumpClangDiagnostics) {
-      llvm::errs() << "clang importer redirected file mappings:\n";
-      for (const auto &mapping : fileMapping.redirectedFiles) {
-        llvm::errs() << "   mapping real file '" << mapping.second
-                     << "' to virtual file '" << mapping.first << "'\n";
-      }
-      llvm::errs() << "\n";
-    }
-
-    if (!fileMapping.overridenFiles.empty()) {
-      llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> overridenVFS =
-          new llvm::vfs::InMemoryFileSystem();
-      for (const auto &file : fileMapping.overridenFiles) {
-        if (importerOpts.DumpClangDiagnostics) {
-          llvm::errs() << "clang importer overriding file '" << file.first
-                       << "' with the following contents:\n";
-          llvm::errs() << file.second << "\n";
-        }
-        auto contents = ctx.Allocate<char>(file.second.size() + 1);
-        std::copy(file.second.begin(), file.second.end(), contents.begin());
-        // null terminate the buffer.
-        contents[contents.size() - 1] = '\0';
-        overridenVFS->addFile(file.first, 0,
-                              llvm::MemoryBuffer::getMemBuffer(StringRef(
-                                  contents.begin(), contents.size() - 1)));
-      }
-      llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> overlayVFS =
-          new llvm::vfs::OverlayFileSystem(VFS);
-      VFS = overlayVFS;
-      overlayVFS->pushOverlay(overridenVFS);
-    }
-  }
-
   // Create a new Clang compiler invocation.
   {
     if (auto ClangArgs = importer->getClangCC1Arguments(ctx, VFS))
diff --git a/lib/ClangImporter/ClangIncludePaths.cpp b/lib/ClangImporter/ClangIncludePaths.cpp
index 0bcb3e4e88e..99393334434 100644
--- a/lib/ClangImporter/ClangIncludePaths.cpp
+++ b/lib/ClangImporter/ClangIncludePaths.cpp
@@ -200,7 +200,7 @@ ClangImporter::createClangArgs(const ClangImporterOptions &ClangImporterOpts,
 }
 
 static SmallVector<std::pair<std::string, std::string>, 2>
-getLibcFileMapping(ASTContext &ctx, StringRef modulemapFileName,
+getLibcFileMapping(const ASTContext &ctx, StringRef modulemapFileName,
                    std::optional<ArrayRef<StringRef>> maybeHeaderFileNames,
                    const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &vfs,
                    bool suppressDiagnostic) {
@@ -266,7 +266,7 @@ getLibcFileMapping(ASTContext &ctx, StringRef modulemapFileName,
 }
 
 static void getLibStdCxxFileMapping(
-    ClangInvocationFileMapping &fileMapping, ASTContext &ctx,
+    ClangInvocationFileMapping &fileMapping, const ASTContext &ctx,
     const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &vfs,
     bool suppressDiagnostic) {
   assert(ctx.LangOpts.EnableCXXInterop &&
@@ -457,7 +457,7 @@ GetPlatformAuxiliaryFile(StringRef Platform, StringRef File,
 }
 
 void GetWindowsFileMappings(
-    ClangInvocationFileMapping &fileMapping, ASTContext &Context,
+    ClangInvocationFileMapping &fileMapping, const ASTContext &Context,
     const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &driverVFS,
     bool &requiresBuiltinHeadersInSystemModules) {
   const llvm::Triple &Triple = Context.LangOpts.Target;
@@ -588,7 +588,7 @@ void GetWindowsFileMappings(
 } // namespace
 
 ClangInvocationFileMapping swift::getClangInvocationFileMapping(
-  ASTContext &ctx, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs,
+  const ASTContext &ctx, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs,
   bool suppressDiagnostic) {
   ClangInvocationFileMapping result;
   if (!vfs)
@@ -658,3 +658,52 @@ ClangInvocationFileMapping swift::getClangInvocationFileMapping(
                          result.requiresBuiltinHeadersInSystemModules);
   return result;
 }
+
+ClangInvocationFileMapping swift::applyClangInvocationMapping(const ASTContext &ctx,
+                                        llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS,
+                                        llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &fileSystem,
+                                        bool suppressDiagnostics) {
+  if (ctx.CASOpts.HasImmutableFileSystem)
+    return ClangInvocationFileMapping();
+
+  ClangInvocationFileMapping fileMapping =
+    getClangInvocationFileMapping(ctx, baseVFS, suppressDiagnostics);
+
+  auto importerOpts = ctx.ClangImporterOpts;
+  // Wrap Swift's FS to allow Clang to override the working directory
+  fileSystem = llvm::vfs::RedirectingFileSystem::create(
+      fileMapping.redirectedFiles, true, *fileSystem);
+  if (importerOpts.DumpClangDiagnostics) {
+    llvm::errs() << "clang importer redirected file mappings:\n";
+    for (const auto &mapping : fileMapping.redirectedFiles) {
+      llvm::errs() << "   mapping real file '" << mapping.second
+                   << "' to virtual file '" << mapping.first << "'\n";
+    }
+    llvm::errs() << "\n";
+  }
+
+  if (!fileMapping.overridenFiles.empty()) {
+    llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> overridenVFS =
+        new llvm::vfs::InMemoryFileSystem();
+    for (const auto &file : fileMapping.overridenFiles) {
+      if (importerOpts.DumpClangDiagnostics) {
+        llvm::errs() << "clang importer overriding file '" << file.first
+                     << "' with the following contents:\n";
+        llvm::errs() << file.second << "\n";
+      }
+      auto contents = ctx.Allocate<char>(file.second.size() + 1);
+      std::copy(file.second.begin(), file.second.end(), contents.begin());
+      // null terminate the buffer.
+      contents[contents.size() - 1] = '\0';
+      overridenVFS->addFile(file.first, 0,
+                            llvm::MemoryBuffer::getMemBuffer(StringRef(
+                                contents.begin(), contents.size() - 1)));
+    }
+    llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> overlayVFS =
+        new llvm::vfs::OverlayFileSystem(fileSystem);
+    fileSystem = overlayVFS;
+    overlayVFS->pushOverlay(overridenVFS);
+  }
+
+  return fileMapping;
+}
diff --git a/lib/DependencyScan/ModuleDependencyScanner.cpp b/lib/DependencyScan/ModuleDependencyScanner.cpp
index 5e0848c7760..b7f7529d881 100644
--- a/lib/DependencyScan/ModuleDependencyScanner.cpp
+++ b/lib/DependencyScan/ModuleDependencyScanner.cpp
@@ -31,6 +31,7 @@
 #include "swift/Frontend/ModuleInterfaceLoader.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
 #include "swift/Subsystems.h"
+#include "clang/Frontend/CompilerInstance.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/SetOperations.h"
 #include "llvm/CAS/CachingOnDiskFileSystem.h"
@@ -267,7 +268,7 @@ ModuleDependencyScanningWorker::ModuleDependencyScanningWorker(
     const SILOptions &SILOptions, ASTContext &ScanASTContext,
     swift::DependencyTracker &DependencyTracker, DiagnosticEngine &Diagnostics)
     : clangScanningTool(*globalScanningService.ClangScanningService,
-                        globalScanningService.getClangScanningFS()) {
+                        globalScanningService.getClangScanningFS(ScanASTContext)) {
   // Create a scanner-specific Invocation and ASTContext.
   workerCompilerInvocation =
       std::make_unique<CompilerInvocation>(ScanCompilerInvocation);
diff --git a/test/ScanDependencies/cxx-overlay-underlying-module-lookup.swift b/test/ScanDependencies/cxx-overlay-underlying-module-lookup.swift
index 26605cd9747..d509ab559c3 100644
--- a/test/ScanDependencies/cxx-overlay-underlying-module-lookup.swift
+++ b/test/ScanDependencies/cxx-overlay-underlying-module-lookup.swift
@@ -3,9 +3,6 @@
 // RUN: %target-swift-frontend -scan-dependencies -o %t/deps.json %s -cxx-interoperability-mode=default -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import
 // RUN: %validate-json %t/deps.json | %FileCheck %s
 
-// rdar://151780437: libstdc++ VFS modulemap redirects not functioning with EBM enabled
-// REQUIRES: OS=macosx
-
 import CxxStdlib
 
 // CHECK: "mainModuleName": "deps"