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

[Dan Klishch via cfe-commits]

DanShaders wrote:

Okay, @mstorsjo @MaskRay, what is the way forward?

Am I right that, as for the user-facing changes, `[[gcc_struct]]` cancelling implicit `-mms-bitfilds` on MinGW is fine and silently ignoring `-m{no-}ms-bitfields` on `windows-msvc` is not? When exactly should we disallow `-m{no-,}ms-bitfields`? Should the aforementioned `--target=x86_64-pc-windows-msvc -fc++-abi=itanium -mms-bitfields` be accepted? Is it fine to provide `[[gcc_struct]]` on MSVC because of reasons I outlined before?

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