gccrs: add unused visibilities lint

Warn on a visibility qualifier applied to a `const _` item, as it has no
effect.

gcc/rust/ChangeLog:

	* checks/lints/unused/rust-unused-checker.cc (UnusedChecker::visit):
	Warn on a visibility qualifier on a `const _` item.

gcc/testsuite/ChangeLog:

	* rust/compile/unused-visibilities_0.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>