analyzer: show saved diagnostics as nodes in .eg.dot dumps

analyzer: show saved diagnostics as nodes in .eg.dot dumps

I've been using this tweak to the output of
-fdump-analyzer-exploded-graph in my working copies for a while;
the extra red nodes make it *much* easier to find the places where
diagnostics are being emitted (or rejected by the diagnostic_manager).

gcc/analyzer/ChangeLog:
	* diagnostic-manager.cc (saved_diagnostic::dump_dot_id): New.
	(saved_diagnostic::dump_as_dot_node): New.
	* diagnostic-manager.h (saved_diagnostic::dump_dot_id): New decl.
	(saved_diagnostic::dump_as_dot_node): New decl.
	* engine.cc (exploded_node::dump_dot): Add nodes for saved
	diagnostics.

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