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

[Dan Klishch via cfe-commits]

DanShaders wrote:

> Therefore I don't think it's too relevant to implement -mno-ms-bitfields for MSVC targets (as I guess it could open a huge can of worms).

I want to only alter the layout of explicit fields in the class with the future MSVC `-mno-ms-bitfields` implementation. This is always save to do without worrying much about other parts of the compiler.

I should stress that regardless of original motivation the additional complexity of `-fms-layout-compatibility` makes checks in a few other places easier to follow.

Also, do we want to support `--target=x86_64-pc-windows-msvc -fc++-abi=itanium -mms-bitfields`? If so, it would be much simpler with my approach.

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