analyzer: convert note_adding_context to annotating_context

This is enabling work towards the context being able to inject
events into diagnostic paths, rather than just notes after the
warning.

gcc/analyzer/ChangeLog:
	* region-model.cc
	(class check_external_function_for_access_attr::annotating_ctxt):
	Convert to an annotating_context.
	* region-model.h (class note_adding_context): Rename to...
	(class annotating_context): ...this, updating the "warn" method.
	(note_adding_context::make_note): Replace with...
	(annotating_context::add_annotations): ...this.

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