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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > Of the three, I lean towards 3 actually, I think that is perhaps the BEST idea, and is perhaps supported by our existing infrastructure already (if you have Attr.td set its targets right?). I'd like to see what Aaron has to say, but I THINK that is my preference baring any concerns.
> 
> Yeah, I agree that that is probably the best option (though we should probably still update the diagnostic I mentioned, because it’s not obvious imo that `[[assume]]` != `[[clang::assume]]`).

Agreed, I think an improved diag there is a good idea.

> 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...
> 
> That and the `[[clang::assume]]` docs should probably point out that this is _not_ C++23’s `[[assume]]` if they don’t do that already.

I might lean toward CXXAssumeAttr and OMPAssumeAttr ? 

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