[Dependency Scanning] Scan embedded header content if file doesn't exist

Fallback to scan embedded header content when the bridging header path
encoded in the swift binary module doesn't exit on the disk.

rdar://144261730

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -36,6 +36,7 @@
 #include "llvm/CAS/CachingOnDiskFileSystem.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/MemoryBufferRef.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/VersionTuple.h"
@@ -1063,6 +1064,42 @@ void ModuleDependencyScanner::resolveHeaderDependenciesForModule(
   if (!isTextualModuleWithABridgingHeader && !isBinaryModuleWithHeaderInput)
     return;
 
+
+  std::optional<std::string> headerPath;
+  std::unique_ptr<llvm::MemoryBuffer> sourceBuffer;
+  std::optional<llvm::MemoryBufferRef> sourceBufferRef;
+
+  auto extractHeaderContent =
+      [&](const SwiftBinaryModuleDependencyStorage &binaryMod)
+      -> std::unique_ptr<llvm::MemoryBuffer> {
+    auto header = binaryMod.headerImport;
+    // Check to see if the header input exists on disk.
+    auto FS = ScanASTContext.SourceMgr.getFileSystem();
+    if (FS->exists(header))
+      return nullptr;
+
+    auto moduleBuf = FS->getBufferForFile(binaryMod.compiledModulePath);
+    if (!moduleBuf)
+      return nullptr;
+
+    auto content = extractEmbeddedBridgingHeaderContent(std::move(*moduleBuf),
+                                                        ScanASTContext);
+    if (content.empty())
+      return nullptr;
+
+    return llvm::MemoryBuffer::getMemBufferCopy(content, header);
+  };
+
+  if (isBinaryModuleWithHeaderInput) {
+    auto &binaryMod = *moduleDependencyInfo.getAsSwiftBinaryModule();
+    if (auto embeddedHeader = extractHeaderContent(binaryMod)) {
+      sourceBuffer = std::move(embeddedHeader);
+      sourceBufferRef = sourceBuffer->getMemBufferRef();
+    } else
+      headerPath = binaryMod.headerImport;
+  } else
+    headerPath = *moduleDependencyInfo.getBridgingHeader();
+
   withDependencyScanningWorker(
       [&](ModuleDependencyScanningWorker *ScanningWorker) {
         auto clangImporter = static_cast<ClangImporter *>(
@@ -1072,12 +1109,9 @@ void ModuleDependencyScanner::resolveHeaderDependenciesForModule(
         std::optional<std::string> includeTreeID;
         std::vector<std::string> bridgingHeaderCommandLine;
         auto headerScan = clangImporter->getHeaderDependencies(
-            moduleID,
-            isTextualModuleWithABridgingHeader
-                ? *moduleDependencyInfo.getBridgingHeader()
-                : moduleDependencyInfo.getAsSwiftBinaryModule()->headerImport,
-            /*sourceBuffer=*/std::nullopt, ScanningWorker->clangScanningTool,
-            cache, headerClangModuleDependencies, headerFileInputs,
+            moduleID, headerPath, sourceBufferRef,
+            ScanningWorker->clangScanningTool, cache,
+            headerClangModuleDependencies, headerFileInputs,
             bridgingHeaderCommandLine, includeTreeID);
         if (!headerScan) {
           // Record direct header Clang dependencies