[clang] [AST] Support structural equivalence checking of attributes on Decls (PR #168769)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Thanks @ChuanqiXu9 for the background! Because this code isn't used by modules for C++ and it's a good improvement for C, I think it's ready to land as-is and we can think about how to reuse the attribute logic later.

https://github.com/llvm/llvm-project/pull/168769