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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This needs attribute documentation and a release note.

Additionally, I think we should prohibit using this in Microsoft mode (that is, diagnose that) rather than having it be a no-op.  We can enable it in the future, but if we permit it, folks will assume it works.

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