[clang] [Modules][Diagnostic] Don't claim a METADATA mismatch is always in PCH file. (PR #101280)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

I have more similar improvements when a diagnostic claims a mismatch is in PCH file while it is in a module file. Those would be separate PR(s) as the code change is quite different.

https://github.com/llvm/llvm-project/pull/101280