[clang] [clang][Sema] Combine fallout warnings to just one warning (PR #127546)

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foxtran wrote:

> I think this might be a good candidate for the new %enum_select thingy that @erichkeane added recently (see https://clang.llvm.org/docs/InternalsManual.html#formatting-a-diagnostic-argument for documentation if you’re not familiar w/ it).

Yep, thank you!

Will it have redefinition problem since I need to share this enum between two diagnostics?

https://github.com/llvm/llvm-project/pull/127546