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

[via cfe-commits]

Sirraide wrote:

> Existing code already uses `-Wreturn-type` though, and this patch is enabling it in all language modes, not just C23 and above.

At least that’s what I hope we’ll be able to do. A compromise would be to only enable it in e.g. C23/C++26/whatever, but that doesn’t quite seem satisfactory to me...

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