analyzer: refactor custom_event, introducing precanned_custom_event class

analyzer: refactor custom_event, introducing precanned_custom_event class

I have followup work where a custom event's description would be better
handled via a vfunc rather that a precanned string, hence this
refactoring to make it easy to add custom_event subclasses.

gcc/analyzer/ChangeLog:
	* checker-path.cc (class custom_event): Make abstract to allow for
	custom vfuncs, splitting existing implementation into...
	(class precanned_custom_event): New subclass.
	(custom_event::get_desc): Move to...
	(precanned_custom_event::get_desc): ...subclass.
	* checker-path.h (class custom_event): Make abstract to allow for
	custom vfuncs, splitting existing implementation into...
	(class precanned_custom_event): New subclass.
	* diagnostic-manager.cc (diagnostic_manager::add_events_for_eedge):
	Use precanned_custom_event.
	* engine.cc
	(stale_jmp_buf::maybe_add_custom_events_for_superedge): Likewise.
	* sm-signal.cc (signal_delivery_edge_info_t::add_events_to_path):
	Likewise.

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