[clang] [modules] Add diagnostic about performed input file validation when encounter unrecoverable changed input file. (PR #180899)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> Silly question, if this error was emitted in a module that was compiled in the same compiler instance, what version of the note would we see? `input file validation has covered all files`? Should that be a special case?

Based on recent memory most likely we would have "was skipped for the current build session" (validation file was written after the beginning of the build session). If module build took less than 1 second, it would be "has covered only user files".

I don't know what special case should be and what would be the purpose. Not saying it's impossible, just personally I have no ideas now.

https://github.com/llvm/llvm-project/pull/180899