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

[Dan Klishch via cfe-commits]

DanShaders wrote:

Ping @MaskRay


Changes in the force-push:
- rebased on top of main;
- moved `defaultsToMsStruct` to `ASTContext` since it also depends on auxiliary target.

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