[clang modules] Setting DebugCompilationDir when it is safe to ignore current working directory

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -492,8 +492,23 @@ static std::string getModuleContextHash(const ModuleDeps &MD,
   auto &FSOpts = const_cast<FileSystemOptions &>(CI.getFileSystemOpts());
   if (CWD && !IgnoreCWD)
     HashBuilder.add(*CWD);
-  else
+  else {
     FSOpts.WorkingDir.clear();
+    auto &CGOpts = const_cast<CodeGenOptions &>(CI.getCodeGenOpts());
+    if (CGOpts.DwarfVersion && CWD) {
+      // It is necessary to explicitly set the DebugCompilationDir
+      // to a common directory (e.g. root) if IgnoreCWD is true.
+      // When IgnoreCWD is true, the module's content should not depend
+      // on the current working directory. However, if dwarf information
+      // is needed (when CGOpts.DwarfVersion is non-zero), and if
+      // CGOpts.DebugCompilationDir is not explicitly set,
+      // the current working directory will be automatically embedded
+      // in the dwarf information in the pcm, contradicting the assumption
+      // that it is safe to ignore the CWD. Thus in such cases,
+      // CGOpts.DebugCompilationDir is explicitly set to a common directory.
+      CGOpts.DebugCompilationDir = llvm::sys::path::root_path(*CWD);
+    }
+  }
 
   // Hash the BuildInvocation without any input files.
   SmallString<0> ArgVec;

clang/test/ClangScanDeps/modules-debug-dir.c

@@ -0,0 +1,30 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: sed -e "s|DIR|%/t|g" %t/cdb.json.in > %t/cdb.json
+// RUN: clang-scan-deps -compilation-database %t/cdb.json -format \
+// RUN:   experimental-full > %t/result.json
+// RUN: cat %t/result.json | FileCheck %s
+
+//--- cdb.json.in
+[{
+  "directory": "DIR",
+  "command": "clang -target x86_64-apple-darwin -c -g -gmodules DIR/tu.c -fmodules -fmodules-cache-path=DIR/cache -IDIR/include/ -fdebug-compilation-dir=DIR -o DIR/tu.o",
+  "file": "DIR/tu.c"
+}]
+
+//--- include/module.modulemap
+module mod {
+  header "mod.h"
+}
+
+//--- include/mod.h
+
+//--- tu.c
+#include "mod.h"
+
+// Check the -fdebug-compilation-dir used for the module is the root
+// directory when current working directory optimization is in effect.
+// CHECK:  "modules": [
+// CHECK: "command-line": [
+// CHECK: "-fdebug-compilation-dir=/",
+// CHECK:  "translation-units": [