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

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Sirraide wrote:

> Is it implemented differently in Clang,

The way `-Wreturn-type` is implemented in Clang is we build a CFG from the AST and then walk that to determine whether all paths return a value, if that’s what you’re asking.

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