[clang] [clang] Implement gcc_struct attribute on Itanium targets (PR #71148)

[Martin Storsjö via cfe-commits]

https://github.com/mstorsjo commented:

I had a brief look over this, and it looks reasonable to me, although I'm not anywhere near competent enough to properly review it from the Clang perspective and approve it.

I don't see any added test that actually exercises the new `gcc_struct` attribute anywhere though - I'd like to see that, in a test that runs with `-mms-bitfields` or similar, and shows what effect the attribute has in that context.

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