[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > @Sirraide, would you be comfortable if you, @erichkeane, and I co-authored an RFC to the community?
> 
> Sure, I’d be happy to help with that as best I can. An RFC makes sense imo seeing as this really impacts more parts of LLVM than just Clang.

Excellent, thanks! I will try to get a draft of the RFC over to you both sometime tomorrow and we can go from there. I'll put it in a Google Doc so we can hopefully collaborate more easily on it.

https://github.com/llvm/llvm-project/pull/81014