analyzer: fix state dumps for return_event [PR122003]

analyzer: fix state dumps for return_event [PR122003]

In the reimplementation of supergraph (r16-6063-g0b786d961d4426) the
class return_event moved from being a subclass of superedge_event to
a subclass of checker_event, thus using checker_event::get_program_state
which returns null.

This stopped e.g. HTML state-graph output showing state at return events
in e.g. event (8) of
https://dmalcolm.fedorapeople.org/gcc/2025-12-15/state-diagram-1.c.html

Fixed by implementing return_event::get_program_state.

gcc/analyzer/ChangeLog:
	PR analyzer/122003
	* checker-event.cc (return_event::get_program_state): New.
	* checker-event.h (return_event::get_program_state): New decl.

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