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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

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

Please update the subject of the PR - this doesn't stub out the attribute, it implements it, if I understand things correctly?

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