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

[via cfe-commits]

Sirraide wrote:

Also, just a heads-up, #127623 is likely going to be merged soon and introduce some merge conflicts for you in `CheckFallThroughForBody()`. The code that that pr touches is just deleted here, so that’s fine I think, but maybe make sure that none of the diag ids are `0` if the function being analysed is a coroutine.

(We generate an implicit `return` statement at the end of every coroutine, and if the coroutine is `noreturn`, then we try to emit a ‘noreturn function should not return’ warning. The correct behaviour is to just not emit a warning here at all in that case)

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