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

[Martin Storsjö via cfe-commits]

mstorsjo 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.

I'm not a GCC developer, but I would not think that GCC would consider this an x86-only feature. It just so happens that (upstream) GCC only supports Windows on x86. But MSVC does their own funky bitfield packing on all architectures - so it seems reasonable to want to be able to match it on all architectures.

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