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

[Sam James via llvm-commits]

thesamesam wrote:

> The compromise is to make it a warning which defaults to an error so users who hit the false positives have an escape hatch.

OK, that's fair enough, I think. I can see it being controversial but I think it's a defensible choice (that I agree with).

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