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

[via cfe-commits]

https://github.com/Sirraide commented:

Ah, thanks for the cleanup. I’m doing something related to this at the moment and this definitely helps with that ;Þ

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).

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