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

[Sam James via llvm-commits]

thesamesam wrote:

> Is that something you've looked into by any chance? If not, it would be worthwhile to know what the fallout is. 

I'm worried about the fallout and I have vague recollections of some early work I did when I assumed we were making this change for the C99 enforcement work, but I don't have specifics. I can look into this but I'd need time -- I can't do that ready for Clang 20.

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