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

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Sirraide wrote:

Aaron and I talked about this for a while, and I’ve now posted an RFC about this subject: <https://discourse.llvm.org/t/rfc-make-wreturn-type-an-error-by-default/85775>

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