[clang][deps] Share FileManager between modules (#115065)

The `FileManager` sharing between module-building `CompilerInstance`s
was disabled a while ago due to `FileEntry::getName()` being unreliable.
Now that we use `FileEntryRef::getNameAsRequested()` in places where it
matters, re-enabling `FileManager` is sound and improves performance of
`clang-scan-deps` by ~6.2%.

Author: jansvoboda11

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -335,7 +335,9 @@ class DependencyScanningAction : public tooling::ToolAction {
 
     ScanInstance.getFrontendOpts().GenerateGlobalModuleIndex = false;
     ScanInstance.getFrontendOpts().UseGlobalModuleIndex = false;
-    ScanInstance.getFrontendOpts().ModulesShareFileManager = false;
+    // This will prevent us compiling individual modules asynchronously since
+    // FileManager is not thread-safe, but it does improve performance for now.
+    ScanInstance.getFrontendOpts().ModulesShareFileManager = true;
     ScanInstance.getHeaderSearchOpts().ModuleFormat = "raw";
     ScanInstance.getHeaderSearchOpts().ModulesIncludeVFSUsage =
         any(OptimizeArgs & ScanningOptimizations::VFS);