Small improvements to DefaultASTVisitor and nr2.0

Small improvements to DefaultASTVisitor and nr2.0

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc
	(DefaultASTVisitor::visit): Only visit the path of an instance
	of Visibility if the instance has a path.
	* ast/rust-ast.h
	(SimplePath::SimplePath): Make sure constructors are explicit.
	* resolve/rust-early-name-resolver-2.0.cc
	(Early::visit_attributes): Pass entire paths to
	NameResolutionContext::resolve_path.
	(Early::visit): Likewise and avoid copying a path.
	* resolve/rust-forever-stack.hxx
	(ForeverStack::resolve_path): Assert that at least one path
	segment has been passed in.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>