[clang] [Clang] disallow # operators in attribute argument lists (PR #147308)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

Actually, the changes still LGTM, but let's hold off on landing the changes until we give WG21 a chance to weigh in. It's possible we'll end up converging and not need to have different behaviors.

https://github.com/llvm/llvm-project/pull/147308