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

[John McCall via cfe-commits]

rjmccall wrote:

> @rjmccall The problem will arise only if GCC implements support for MSVC C++ ABI and decides that there is a better way to implement `gcc_struct`. Since, AFAIC, MSVC-compatibility for GCC is not even planned, it's unlikely anybody there will have strong opinions on this. Yet I posted a question on gcc mailing list (https://gcc.gnu.org/pipermail/gcc/2023-December/242963.html) and, unsurprisingly, got no replies in a week.

My question is actually more about supporting `ms_struct` and `gcc_struct` on other architectures than x86 than it's about MSVC-compatible targets.  I agree that it's highly unlikely that GCC would ever implement the MSVC C++ ABI, but e.g. MinGW on ARM doesn't seem out of the picture.

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