gccrs: expand: Add inner attribute expansion stubs

gccrs: expand: Add inner attribute expansion stubs

Add function stubs for inner attribute proc macros expansion.

gcc/rust/ChangeLog:

	* expand/rust-expand-visitor.cc (ExpandVisitor::visit): Add call
	to inner attribute stub.
	(ExpandVisitor::expand_inner_attribute): Expand one single
	attribute.
	(ExpandVisitor::visit_inner_using_attrs): Expand a given item
	using a vector of attributes.
	(ExpandVisitor::visit_inner_attrs): Visit inner attributes of a
	given item.
	* expand/rust-expand-visitor.h: Add function prototypes.

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