[clang] Fix codegen of consteval functions returning an empty class, and related issues (PR #93115)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Is this worth back porting as it is a bugfix over code gen?

IMO, it's worth considering, but if we want to go down this route, I think we need to do so relatively quickly -- we have about two weeks until rc3, and given the size of this change, I'm not certain we should try landing it any later than rc3 just due to risk.

https://github.com/llvm/llvm-project/pull/93115