[clang] [clang][modules] Timestamp PCM files when writing (PR #112452)

[Volodymyr Sapsai via cfe-commits]

================
@@ -4905,6 +4905,10 @@ ASTFileSignature ASTWriter::WriteAST(Sema &SemaRef, StringRef OutputFile,
   this->BaseDirectory.clear();
 
   WritingAST = false;
+
+  if (WritingModule)
+    updateModuleTimestamp(OutputFile);
----------------
vsapsai wrote:

Don't have a strong opinion about the location of updating the timestamp. 

To be consistent with another usage of `updateModuleTimestamp` shouldn't you check
```c++
HeaderSearchOptions &HSOpts = PP.getHeaderSearchInfo().getHeaderSearchOpts();
if (HSOpts.ModulesValidateOncePerBuildSession)
```

? And I hope it can help with the explicitly-built .pcm files.

https://github.com/llvm/llvm-project/pull/112452