analyzer: add ability for context to add events to a saved_diagnostic

gcc/analyzer/ChangeLog:
	* diagnostic-manager.cc (saved_diagnostic::add_event): New.
	(saved_diagnostic::add_any_saved_events): New.
	(diagnostic_manager::add_event): New.
	(dedupe_winners::emit_best): New.
	(diagnostic_manager::emit_saved_diagnostic): Make "sd" param
	non-const.  Call saved_diagnostic::add_any_saved_events.
	* diagnostic-manager.h (saved_diagnostic::add_event): New decl.
	(saved_diagnostic::add_any_saved_events): New decl.
	(saved_diagnostic::m_saved_events): New field.
	(diagnostic_manager::add_event): New decl.
	(diagnostic_manager::emit_saved_diagnostic): Make "sd" param
	non-const.
	* engine.cc (impl_region_model_context::add_event): New.
	* exploded-graph.h (impl_region_model_context::add_event): New decl.
	* region-model.cc
	(noop_region_model_context::add_event): New.
	(region_model_context_decorator::add_event): New.
	* region-model.h (region_model_context::add_event): New vfunc.
	(noop_region_model_context::add_event): New decl.
	(region_model_context_decorator::add_event): New decl.

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