analyzer: fixes to -fdump-analyzer-json

I've been implementing a PyGTK viewer for the output of
-fdump-analyzer-json, to help me debug analyzer issues:
  https://github.com/davidmalcolm/gcc-analyzer-viewer
The viewer is very much just a work in progress.

This patch adds some fields that were missing from the dump, and
fixes some mistakes I spotted whilst working on the viewer.

gcc/analyzer/ChangeLog:
	* engine.cc (strongly_connected_components::to_json): New.
	(worklist::to_json): New.
	(exploded_graph::to_json): JSON-ify the worklist.
	* exploded-graph.h (strongly_connected_components::to_json): New
	decl.
	(worklist::to_json): New decl.
	* store.cc (store::to_json): Fix comment.
	* supergraph.cc (supernode::to_json): Fix reference to
	"returning_call" in comment.  Add optional "fun" to JSON.
	(edge_kind_to_string): New.
	(superedge::to_json): Add "kind" to JSON.