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

[Jeremy Rifkin via cfe-commits]

jeremy-rifkin wrote:

Hi @foxtran, I have been working on a proposal on the C++ side. Extensive justification is needed to justify breakage here and I have been attempting to survey the extent of potential breakage. The wording is also tricky especially since attributes are designed to be ignorable. Would you be interested in collaborating on a paper?

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