[clang][Dependency Scanning] Move Module Timestamp Update After Compilation Finishes (#151774)

When two threads are accessing the same `pcm`, it is possible that the
reading thread sees the timestamp update, while the file on disk is not
updated.

This PR moves timestamp update from `writeAST` to
`compileModuleAndReadASTImpl`, so we only update the timestamp after the
file has been committed to disk.

rdar://152097193

Author: qiongsiwu

clang/lib/Frontend/CompilerInstance.cpp

@@ -1485,6 +1485,14 @@ static bool compileModuleAndReadASTImpl(CompilerInstance &ImportingInstance,
     return false;
   }
 
+  // The module is built successfully, we can update its timestamp now.
+  if (ImportingInstance.getPreprocessor()
+          .getHeaderSearchInfo()
+          .getHeaderSearchOpts()
+          .ModulesValidateOncePerBuildSession) {
+    ImportingInstance.getModuleCache().updateModuleTimestamp(ModuleFileName);
+  }
+
   return readASTAfterCompileModule(ImportingInstance, ImportLoc, ModuleNameLoc,
                                    Module, ModuleFileName,
                                    /*OutOfDate=*/nullptr, /*Missing=*/nullptr);

clang/lib/Serialization/ASTWriter.cpp

@@ -5461,11 +5461,6 @@ ASTWriter::WriteAST(llvm::PointerUnion<Sema *, Preprocessor *> Subject,
 
   WritingAST = false;
 
-  if (WritingModule && PPRef.getHeaderSearchInfo()
-                           .getHeaderSearchOpts()
-                           .ModulesValidateOncePerBuildSession)
-    ModCache.updateModuleTimestamp(OutputFile);
-
   if (ShouldCacheASTInMemory) {
     // Construct MemoryBuffer and update buffer manager.
     ModCache.getInMemoryModuleCache().addBuiltPCM(