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

[Erich Keane via cfe-commits]

erichkeane wrote:

> If we can come to a consensus as to what to do w/ `clang::assume` and `__attribute__((assume))` reasonably soon, then I’ll handle `[[omp::assume]]` in this pr as well; otherwise, I’ll open a pr for `[[omp::assume]]` tomorrow or so.

This patch is big enough as it is, as far as LOC and what effect, I would prefer we split the OMP decisions off to a separate patch and discuss that /implemetn that there.

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