[clang] [Modules][Diagnostic] Improve diagnostics for stale module dependencies (PR #167713)

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devajithvs wrote:

> I don't think you can say whether it's the importer or importee that's out-of-date at this point.

@jansvoboda11 Do you have a suggestion on what to do here?

The existing diagnostic is misleading in common workflows because it asks users to rebuild the importee (A), even when A was just rebuilt and rebuilding it again won’t resolve the mismatch. We need to somehow point to the importer (B) if that is the module that needs rebuilding.

Maybe we can adjust the wording to avoid implying causality, "module file ‘B.pcm’ is incompatible with the current version of ‘A.pcm’". This message might not be misleading but wouldn't say which module needs rebuilding.

https://github.com/llvm/llvm-project/pull/167713