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

[Aaron Ballman via cfe-commits]

AaronBallman 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/C26. So, existing packages should not be affected significantly. :-)

Existing code already uses `-Wreturn-type` though, and this patch is enabling it in all language modes, not just C23 and above.

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