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

[Cyndy Ishida via cfe-commits]

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@@ -29,6 +29,10 @@ def note_ast_file_rebuild_required
     : Note<"please rebuild precompiled file '%0'">;
 def note_module_cache_path : Note<
     "after modifying system headers, please delete the module cache at '%0'">;
+def note_ast_file_input_files_validation_status : Note<"input file validation "
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cyndyishida wrote:

Looking at the message next to the error, it begs the question. _"If input file validation is disabled/skipped/already done, why would the compiler notice the file is out of date?"_
Maybe it's alright, this is an IYKYK sort of note, but maybe prepending an `earlier input file validation is disabled...` would help clarify?

Also, you should change `is disabled for this kind of module` to `is disabled for this kind of precompiled file` or allow deciding at the callsite if it's a module or pch. 

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