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

[John McCall via cfe-commits]

rjmccall wrote:

Right, I'd just like to make sure that we're not deepening a divergence here.  It would be good to get agreement from the GCC devs that they think `ms_struct` probably ought to do something on e.g. ARM MinGW targets and that they consider this a bug (in a feature that they may not really support, which is fine).  But if they think *we're* wrong and that this really should only have effect on x86, I would like to know that.

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