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

[Aaron Ballman via cfe-commits]

AaronBallman 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).

Yeah, my primary concern in terms of fallout is the behavior of `autoconf`. It'd be nice if we could understand the damage there before landing, but now that we're early in the 21.x cycle, we could land and see how it impacts early adopters.

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