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

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foxtran wrote:

> No, that's not the principal issue (but you should consider building userland with it to see how many packages will need fixing)

Firstly, they need to start using C23. So, existing packages should not be affected significantly.  :-)

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