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

[Dan Klishch via cfe-commits]

DanShaders wrote:

Hmm, but `ms_struct` has been accepted on non-x86 for at least 5 years now. The PR merely adds a per-structure opt out toggle for it and, honestly, I don't see how GCC might implement it differently. 

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