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

[Sam James via llvm-commits]

thesamesam wrote:

@AaronBallman On another note, we've had people request this many times in GCC for C++ at least, but it has FPs as a result of how it's implemented. I was under the impression Clang generally didn't do "middle-end"-style (or optimisation-dependent) warnings. Is it implemented differently in Clang, am I misremembering or misunderstanding something, or is Clang just accepting the FPs here?

(cc @pinskia)

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