[clang] [clang] Stub out gcc_struct attribute (PR #71148)

[John McCall via cfe-commits]

rjmccall wrote:

It sounds like we're talking about extending the behavior of these attributes, and I'd like to make sure that whatever we do here is acceptable to GCC.  That is, if we're going to start accepting these attributes on new targets which GCC doesn't currently support, we should make sure GCC agrees that these attributes should have that effect on those targets, in case GCC ever supports those targets.

I agree that it would make the most sense to stage this as two separate patches, one to just add `gcc_struct` in the places that GCC does currently, and another to support `ms_struct` and `gcc_struct` in more places.

https://github.com/llvm/llvm-project/pull/71148