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

[Sam James via llvm-commits]

thesamesam wrote:

Thanks. I expect there'll be some fallout but you also get missed optimisation bugs out of that in some cases, so.

>  Another option would be to make this default to an error depending on the language mode (e.g. only in C23 or later, and either unconditionally in C++ or in e.g. C++17 or later etc.); I don’t have any actual data on this, but I would imagine that most programs that actually have code that triggers this warning are probably C99 or earlier.

This unfortunately won't work when we change the default to C23 and so on.

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