I have followup work that touches this, so it's easiest to get this
cleanup in first.

gcc/analyzer/ChangeLog:
	* engine.cc (exploded_node::exploded_node): Move implementation
	here from header; accept point_and_state by const reference rather
	than by value.
	* exploded-graph.h (exploded_node::exploded_node): Pass
	point_and_state by const reference rather than by value.  Move
	body to engine.cc.