[clang][deps] Do not cache PCM files

[jansvoboda11]

Summary

On incremental scan, caching an out-of-date PCM on the VFS layer causes each TU and each module to recompile the PCM again. This is huge performance problem. Stop caching ".pcm" files.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

return scanForDirectivesIfNecessary(*MaybeEntry, Filename,
       .unwrapError();
 }
 
 llvm::ErrorOr<llvm::vfs::Status>
 DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   SmallString<256> OwnedFilename;
   StringRef Filename = Path.toStringRef(OwnedFilename);
 
+  if (Filename.endswith(".pcm"))
+    return getUnderlyingFS().status(Path);
+
   llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
   if (!Result)
     return Result.getError();
   return Result->getStatus();
 }
 
 namespace {
 
return llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>(
       std::unique_ptr<llvm::vfs::File>(std::move(Result)));
 }
 
 llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
 DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
   SmallString<256> OwnedFilename;
   StringRef Filename = Path.toStringRef(OwnedFilename);
 
+  if (Filename.endswith(".pcm"))
+    return getUnderlyingFS().openFileForRead(Path);
+
   llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
   if (!Result)
     return Result.getError();
   return DepScanFile::create(Result.get());
 }