gccrs: Make expand visitor inherit from default visitor

gccrs: Make expand visitor inherit from default visitor

Many visit functions in the expand visitor simply visit their components
like the default visitor. Making the expand visitor inherit from the
default visitor allows us to keep all visitor in sync without having to
change every visitor.

gcc/rust/ChangeLog:

	* expand/rust-expand-visitor.cc (ExpandVisitor::go): Add call to visit
	on the crate.
	(ExpandVisitor::visit): Remove some visit functions in favor of their
	default visitor counterpart.
	* expand/rust-expand-visitor.h (class ExpandVisitor): Inherit from
	default visitor and remove now useless function prototypes.

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