Make feature gate visitor inherit from default one

Make feature gate visitor inherit from default one

The feature gating behavior may be shortened and kept cleaner using the
default visitor. This means less maintenance on visit functions as the
traversal is shared by multiple visitors.

gcc/rust/ChangeLog:

	* checks/errors/rust-feature-gate.cc (FeatureGate::visit): Add a visit
	function for the crate level.
	(FeatureGate::check): Add call to crate visit.
	* checks/errors/rust-feature-gate.h (class FeatureGate): Remove now
	useless visit functions (traversal only).

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>