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

[via cfe-commits]

Sirraide wrote:

> 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

That also reminds me: A while ago I ran into a bug where someone had added a few cases to one diagnotic but forgot to update another diagnostic that also needed those cases, and we ended up asserting because of that; if we require every diagnostic that uses `%enum_select` with the same enum to have the exact same number of cases then that might help avoid such bugs in the future.

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