analyzer: use label_text for superedge::get_description

analyzer: use label_text for superedge::get_description

gcc/analyzer/ChangeLog:
	* checker-path.cc (start_cfg_edge_event::get_desc): Update for
	superedge::get_description returning a label_text.
	* engine.cc (feasibility_state::maybe_update_for_edge): Likewise.
	* supergraph.cc (superedge::dump): Likewise.
	(superedge::get_description): Convert return type from char * to
	label_text.
	* supergraph.h (superedge::get_description): Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>