GCC/clang warning incompatibility with unused private member variables

[Jakub Jelinek]

On Fri, Jun 11, 2021 at 04:03:34PM -0400, Jason Merrill via Gcc wrote:
> You can use #pragma to disable a warning for a particular section of code:
> 
> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wattributes"
> class Test {
>      [[maybe_unused]] int a_;
>      void b() {};
> };
> #pragma GCC diagnostic pop
> 
> But I also agree that GCC shouldn't warn here.

We could do that by using a wrapper around handle_unused_attribute
for the maybe_unused attribute, that way warn on unused attribute on
FIELD_DECLs, but not for maybe_unused (until we actually implement some
warning that uses it).

	Jakub