[clang][deps] stop reusing FileManager in DependencyScanningAction

[Kale]

Summary

DepFS already provides necessary caching, so explicitly stop reusing the FileMgr for possibly cache conflict.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

 //===- DependencyScanningWorker.cpp - clang-scan-deps worker --------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/Tooling/DependencyScanning/DependencyScanningWorker.h"
+#include "clang/Basic/FileManager.h"
 #include "clang/CodeGen/ObjectFilePCHContainerOperations.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Frontend/Utils.h"
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
if (DepFS) {
         DepFS->disableMinimization(File.getKey());
       // Don't minimize any files that were explicitly passed in the build
       // settings and that might be opened.
       for (const auto &E : ScanInstance.getHeaderSearchOpts().UserEntries)
         DepFS->disableMinimization(E.Path);
       for (const auto &F : ScanInstance.getHeaderSearchOpts().VFSOverlayFiles)
         DepFS->disableMinimization(F);
 
-      // Support for virtual file system overlays on top of the caching
-      // filesystem.
-      FileMgr->setVirtualFileSystem(createVFSFromCompilerInvocation(
-          ScanInstance.getInvocation(), ScanInstance.getDiagnostics(), DepFS));
+      // DepFS already provides caching, so we don't need to reuse FileManager.
+      // The provided FileManger is abandoned for possibly invalid caches caused
+      // by the change of VFS.
+      auto VFS = createVFSFromCompilerInvocation(
+          ScanInstance.getInvocation(), ScanInstance.getDiagnostics(), DepFS);
+      FileMgr = new FileManager(FileMgr->getFileSystemOpts(), VFS);
+      ScanInstance.setFileManager(FileMgr);
+      ScanInstance.createSourceManager(*FileMgr);
 
       // Pass the skip mappings which should speed up excluded conditional block
       // skipping in the preprocessor.
       ScanInstance.getPreprocessorOpts()
           .ExcludedConditionalDirectiveSkipMappings = &PPSkipMappings;
     }
 
     // Create the dependency collector that will collect the produced