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

[Erich Keane via cfe-commits]

erichkeane wrote:

We'd need to see what @alexey-bataev has to say about renaming the OMP assume attribute.  I imagine that'll end up causing a headache for his users.

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