[clang] [clang-tools-extra] [Clang] [Sema] Make -Wreturn-type an error by default (PR #131207)

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Sirraide wrote:

I think we *can* try and make this change now, but the question is should we? I recall @AaronBallman having some concerns about autoconf.

(Also, yes, I will merge main again before this is merged because I wouldn’t be surprised if more tests got added in the meantime that will end up failing because of this.)

https://github.com/llvm/llvm-project/pull/131207