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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> > Have you checked the performance for incremental builds?
> 
> I haven't, since I believe this change doesn't affect incremental builds at all.

Fair enough. I was thinking that you can achieve the same improvement by using some contentionless-update-mtime function. But I haven't found any and `futimens` doesn't seem to help (at least on my hardware in my testing).

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