[clang][DependencyScanning] Unify initVFS helpers to return OverlayFileSystem (NFC) (#172166)

This patch is part of a series of splitting the work from #169964 into
smaller changes (see review linked below).

As part of this work, the regular and by-name dependency scanning APIs
need to be extended to allow the VFS to be initialized via the
initVFS helpers before invoking the scanning API, with the resulting
overlay filesystem passed into the worker instead.

To make the newly added API changes consistent, this patch updates both
helpers to return an OverlayFileSystem.

#169964 (review)

Author: naveen-seth

clang/include/clang/DependencyScanning/DependencyScannerImpl.h

@@ -98,7 +98,8 @@ std::unique_ptr<CompilerInvocation>
 createCompilerInvocation(ArrayRef<std::string> CommandLine,
                          DiagnosticsEngine &Diags);
 
-std::pair<IntrusiveRefCntPtr<llvm::vfs::FileSystem>, std::vector<std::string>>
+std::pair<IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem>,
+          std::vector<std::string>>
 initVFSForTUBufferScanning(IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS,
                            ArrayRef<std::string> CommandLine,
                            StringRef WorkingDirectory,

clang/lib/DependencyScanning/DependencyScannerImpl.cpp

@@ -430,15 +430,15 @@ dependencies::createCompilerInvocation(ArrayRef<std::string> CommandLine,
   return Invocation;
 }
 
-std::pair<IntrusiveRefCntPtr<llvm::vfs::FileSystem>, std::vector<std::string>>
+std::pair<IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem>,
+          std::vector<std::string>>
 dependencies::initVFSForTUBufferScanning(
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS,
     ArrayRef<std::string> CommandLine, StringRef WorkingDirectory,
     llvm::MemoryBufferRef TUBuffer) {
   // Reset what might have been modified in the previous worker invocation.
   BaseFS->setCurrentWorkingDirectory(WorkingDirectory);
 
-  IntrusiveRefCntPtr<llvm::vfs::FileSystem> ModifiedFS;
   auto OverlayFS =
       llvm::makeIntrusiveRefCnt<llvm::vfs::OverlayFileSystem>(BaseFS);
   auto InMemoryFS = llvm::makeIntrusiveRefCnt<llvm::vfs::InMemoryFileSystem>();
@@ -449,11 +449,10 @@ dependencies::initVFSForTUBufferScanning(
   IntrusiveRefCntPtr<llvm::vfs::FileSystem> InMemoryOverlay = InMemoryFS;
 
   OverlayFS->pushOverlay(InMemoryOverlay);
-  ModifiedFS = OverlayFS;
   std::vector<std::string> ModifiedCommandLine(CommandLine);
   ModifiedCommandLine.emplace_back(InputPath);
 
-  return std::make_pair(ModifiedFS, ModifiedCommandLine);
+  return std::make_pair(OverlayFS, ModifiedCommandLine);
 }
 
 std::pair<IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem>,