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

[via cfe-commits]

Sirraide wrote:

> We have 2 options:
> 
> * Wait for llvm to get improve their handling of assumption based optimizations.
> * Proceed with this work hoping this encourages work on the optimizer.

The standard also mentions that `__has_cpp_attribute` should return `0` ‘if an implementation does not attempt to deduce any such information from assumptions’ [dcl.attr.assume], so another option would be to just have it set to `0` for the time being and just not emit anything until that’s not an issue anymore.

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