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

[via llvm-commits]

cor3ntin wrote:

Thanks for working on that
Did you do a stage 2 build?
I think this is probably Clang 21 material (unless we become sufficiently confident before RC3)

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