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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > > We also need to figure out what to do w/ `clang::assume`, because the entire situation with the two being separate attributes is a bit of a mess, as @erichkeane also pointed out. That can probably be resolved separately from the RFC, however—though I’m not that familiar w/ `clang::assume`, so I may be wrong about this.
> > 
> > 
> > Yeah, I'd really like to figure that one out a bit, but I'm still thinking about it. This RFC is going to be our 'first order' issue though, so if we can get that solved first, we can figure out whether this is something we need to do, or can defer.
> 
> I’d imagine the RFC and the ensuing discussion are gonna take a bit anyway, so we ought to have some time to think about this in the meantinme

Exactly my thoughts. 

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