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

[via cfe-commits]

Sirraide wrote:

Also, `[[assume]]` is currently called `AssumeAttr`, whereas `[[clang::assume]]` is called `AssumptionAttr`, so we might want to rename one of those (e.g. the former to `CXXAssumeAttr` or the latter to `ClangAssumptionAttr` or sth like that) because that sounds like a possible source of confusion in the future...

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