[[gcc_struct]] potential clang compatibility concerns

[Dan Klishch <daklishch@gmail.com>]

Hi,

In the discussion of LLVM's PR adding `[[gnu::gcc_struct]]` support to Clang
(https://github.com/llvm/llvm-project/pull/71148), maintainers asked
me to make sure that whatever
is done there, makes sense for GCC too.

To summarize the long discussion on GitHub, GCC supports gcc_struct,
ms_struct, and
`-m{no-,}ms-bitfields` only on X86, while Clang currently supports ms_struct and
`-m{no-,}ms-bitfields` on all targets with Itanium C++ ABI.
Correspondingly, my PR adds support for
gcc_struct for all targets with the Itanium C++ ABI and paves the road
for gcc_struct and ms_struct
support on targets with Microsoft C++ ABI (mainly,
x86_64-pc-windows-msvc). There, I envision
`ms_struct` to be a no-op (just like `gcc_struct` is usually a no-op
with Itanium C++ ABI) and
`gcc_struct` to change layout of C structs (or fields within C++
classes) to be compatible with the
GenericItanium C++ ABI.

As far as I can tell, the maintainer's question is "in a theoretical
event GCC starts supporting
Microsoft C++ ABI, would it make sense to implement gcc_struct and
ms_struct on it just like I
propose to?".

Thanks,
Dan Klishch