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

[Dan Klishch via cfe-commits]

DanShaders wrote:

@rjmccall Re asking GCC developers about gcc_struct on non-x86: https://gcc.gnu.org/pipermail/gcc/2024-January/243154.html. Either GCC devs aren't really worried about this or I can't properly write emails (what's totally possible). 

@MaskRay Any chance you can look at this PR again? Will you be more happy if instead of `-fms-layout-compatibility` there will be a copy of  `TargetInfo::defaultsToMsStruct` in the driver?

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