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

[Dan Klishch via cfe-commits]

DanShaders wrote:

@MaskRay, I figured the alternative solution with the completely decoupled driver/frontend flags might actually be better than my initial approach. The only issue I see with it is that it requires `s/-mms-bitfields/-fms-layout-compatibility=microsoft/g` in quite a large number of tests which invoke frontend directly.

Would you mind taking a look at this again?

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