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

[Shafik Yaghmour via llvm-commits]

https://github.com/shafik commented:

I see a lot of diagnostics were updated to `DefaultError` but I don't see matching tests that demonstrate the now error. We should be testing each diagnostic. 

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