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

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Sirraide wrote:

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

Hmm, that’s a good point. I think in that case, for now just use `%enum_select` for one of the two and make the other one a regular `%select` (you can still use the enum for both then you emit the diagnostic); just make sure the `%select` cases are in the same order in both diagnostics.

CC @AaronBallman @erichkeane Would it make sense to have proper support for reusing enums across diagnostics w/ `%enum_select`? I can think of a few other diagnostics that have more or less an equivalent set of cases. Maybe we can just emit the enum once and check that every diagnostic that uses  `%enum_select` w/ the same enum has the same number of cases / the same enumerators?



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