analyzer: port pop_frame_callbacks to pub/sub

analyzer: port pop_frame_callbacks to pub/sub

More simplification/consolidation of some callback logic in analyzer in
favor of using the analyzer pub/sub channel.

No functional change intended.

gcc/analyzer/ChangeLog:
	* common.h (struct on_frame_popped): New.
	(subscriber::on_message): New vfunc for on_frame_popped.
	* region-model.cc: Include "context.h" and "channels.h".
	(region_model::pop_frame_callbacks): Delete.
	(region_model::pop_frame): Port from notify_on_pop_frame to
	using pub/sub channel.
	* region-model.h (pop_frame_callback): Delete typedef.
	(region_model::register_pop_frame_callback): Delete.
	(region_model::pop_frame_callbacks): Delete.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/analyzer_cpython_plugin.cc
	(cpython_analyzer_events_subscriber::on_message): Implement for
	on_frame_popped.
	(plugin_init): Drop call to
	region_model::register_pop_frame_callback in favor of the above
	pub/sub handler.

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