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

[Dan Klishch via cfe-commits]

DanShaders wrote:

One more thing. Re binary compatibility concerns: `-mno-ms-bitfields` on MinGW is an equally-sized footgun as on MSVC. Without proper header annotation with `#pragma ms_struct on`, either of them will silently make an ABI mismatch. However, for some reason, supporting `-mno-ms-bitfields` on MinGW is not argued upon.

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