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

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foxtran wrote:

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

Unfortunately, yes... 

https://github.com/llvm/llvm-project/blob/785a5b4676e7aa77904babb9f66e862b5fc39295/clang/test/TableGen/select-enum-errors.td#L15-L20

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