[clang] [clang][modules] Minor improvements to diagnosing `out of date` errors (PR #136612)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> > What would happen if both A-1.pcm and A-2.pcm are built for the same files (e.g., from Sysroot) but have different file names? Not saying it is a valid use case to support, just curious what would happen.
> 
> I tried this, where basically `A-1.pcm` is built again but with `SomethingRandom-1.pcm` and passed that to the final client (B still depends on A-1.pcm). The compilation passed, so I think that means clang doesn't care about the name of the file.

Thanks for the experiment.

https://github.com/llvm/llvm-project/pull/136612