[clang] [compiler-rt] [Clang] Make `-Wreturn-type` default to an error in all language modes (PR #123470)

[via llvm-commits]

Sirraide wrote:

> I see a lot of diagnostics were updated to `DefaultError` but I don't see matching tests that demonstrate the now error. We should be testing each diagnostic.

#123464 also added `-Werror=return-type` to some tests; we can remove those now, which should cover at least some of the diagonstics; I’ll add a test for whatever is left after that because I don’t think we have one for e.g. the coroutine ones.

https://github.com/llvm/llvm-project/pull/123470