[clang] [clang][modules] Minor improvements to diagnosing `out of date` errors (PR #136612)

[Volodymyr Sapsai via cfe-commits]

https://github.com/vsapsai commented:

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.

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