[clang] [Clang] prevent constexpr crash on invalid overrides (PR #184048)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I think this seems reasonable.  A better solution would be to try to recover 'nicer' though.

IF we were to adjust the return type/attributes/exception spec to be 'right' via fixup (or even just adjusting it after creating the error), we might cascade diagnostics more effectly.  But if @AaronBallman is ok with this, so am I.

https://github.com/llvm/llvm-project/pull/184048