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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman I’ve moved the diagnostic under the `-Wattribute-preprocessor-tokens` warning flag and made it a `pedantic` diagnostic, which will be triggered in C++. Is that the behavior you were aiming for?

Yup, this matches what I was thinking, thank you!

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