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

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 requested changes to this pull request.

In general, I don't think you can say whether it's the importer or importee that's out-of-date at this point. With implicit modules without strict context hash, you can get hash collisions, so it very well might be that both the importer and importee are up-to-date, it's just that the importee got recently rebuilt with a different context.

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