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

[via llvm-commits]

Sirraide wrote:

> 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

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.

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