gccrs: Some assorted tweaks and bug fixes

gccrs: Some assorted tweaks and bug fixes

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc
	(DefaultASTVisitor::visit): Visit the loop labels of
	WhileLetLoopExpr instances before visiting their scrutinee
	expressions.
	* resolve/rust-early-name-resolver-2.0.cc
	(Early::resolve_glob_import): Pass the glob import's path
	directly to NameResolutionContext::resolve_path.
	* resolve/rust-toplevel-name-resolver-2.0.cc
	(TopLevel::visit): Remove unnecessary call to
	Identifier::as_string.
	(flatten_glob): Improve handling of cases where a glob use tree
	has no path.

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