[clang] [Clang] Allow the use of [[gnu::visibility]] with #pragma clang attribute (PR #145653)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @AaronBallman Are any of these concerns specific to `[[gnu::visibility]]`? The exact same concerns seem to apply to any other attribute that applies to functions.

We just talked about it between the two of us, and I think I agree, we want to make sure the behavior is 'sane' for these cases and make sure we know they aren't surprising.  Visibility has seemingly a custom appertainment which is concerning.  So please make sure these are covered in tests, and we can make sure we are OK with them.

https://github.com/llvm/llvm-project/pull/145653