Set clang module directory correctly with fmodule-file-home-is-cwd

When building and importing modules built with
`-fmodule-file-home-is-cwd` we should be setting the debug info
directory to the current working directory. This matches the
behavior in clang: https://reviews.llvm.org/D134911

lib/IRGen/IRGenDebugInfo.cpp

@@ -48,6 +48,8 @@
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Frontend/CompilerInstance.h"
+#include "clang/Lex/HeaderSearchOptions.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Serialization/ASTReader.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Config/config.h"
@@ -404,7 +406,7 @@ public:
     auto *CS = DS->InlinedCallSite;
     if (!CS)
       return nullptr;
- 
+
     auto CachedInlinedAt = InlinedAtCache.find(CS);
     if (CachedInlinedAt != InlinedAtCache.end())
       return cast<llvm::MDNode>(CachedInlinedAt->second);
@@ -769,6 +771,15 @@ private:
     uint64_t Signature =
       Desc.getSignature() ? Desc.getSignature().truncatedValue() : ~1ULL;
 
+    // Clang modules using fmodule-file-home-is-cwd should have their
+    // include path set to the working directory.
+    auto &HSI =
+        CI.getClangPreprocessor().getHeaderSearchInfo().getHeaderSearchOpts();
+    if (HSI.ModuleFileHomeIsCwd) {
+      Desc = ASTSourceDescriptor(Desc.getModuleName(), Opts.DebugCompilationDir,
+                                 Desc.getASTFile(), Desc.getSignature());
+    }
+
     // Handle Clang modules.
     if (ClangModule) {
       llvm::DIModule *Parent = nullptr;
@@ -1308,7 +1319,7 @@ private:
     }
     // FIXME: assert that SizeInBits == OffsetInBits.
     SizeInBits = OffsetInBits;
-   
+
     auto FwdDecl = llvm::TempDINode(DBuilder.createReplaceableCompositeType(
         llvm::dwarf::DW_TAG_structure_type, MangledName, Scope, MainFile, 0,
         llvm::dwarf::DW_LANG_Swift, SizeInBits, AlignInBits, Flags,
@@ -1561,7 +1572,7 @@ private:
       return createPointerSizedStruct(Scope,
                                       MangledName,
                                       MainFile, 0, Flags, MangledName);
-      
+
     case TypeKind::BuiltinTuple:
       llvm_unreachable("BuiltinTupleType should not show up here");
 
@@ -1818,7 +1829,7 @@ private:
     return true;
   }
 #endif
-  
+
   llvm::DIType *getOrCreateType(DebugTypeInfo DbgTy) {
     // Is this an empty type?
     if (DbgTy.isNull())

test/DebugInfo/module-file-home-is-cwd.swift

@@ -0,0 +1,16 @@
+// First populate the module cache to avoid issues with implicit modules
+// and -fmodule-file-home-is-cwd.
+// RUN: %target-swift-frontend -module-cache-path %t.mcp -emit-ir %s \
+// RUN:   -g -I %S/Inputs -o /dev/null
+
+// RUN: %target-swift-frontend -module-cache-path %t.mcp -emit-ir %s \
+// RUN:   -g -I %S/Inputs -file-compilation-dir /CWD \
+// RUN:   -Xcc -Xclang -Xcc -fmodule-file-home-is-cwd \
+// RUN:   -o - | %FileCheck %s
+
+import ClangModule.SubModule
+
+let _ = someFunc(0)
+
+// Ensure compilation directory is set to the debug compilation directory.
+// CHECK: !DIFile(filename: "ClangModule", directory: "/CWD")