analyzer: support reverse direction in shortest-paths.h

analyzer: support reverse direction in shortest-paths.h

This patch generalizes shortest-path.h so that it can be used to
find the shortest path from each node to a given target node (on top
of the existing support for finding the shortest path from a given
origin node to each node).

I've marked this as "analyzer" as this is the only code using
shortest-paths.h.

This patch is required by followup work to fix PR analyzer/96374.

gcc/analyzer/ChangeLog:
	* diagnostic-manager.cc (epath_finder::epath_finder):
	Update shortest_paths init for new param.

gcc/ChangeLog:
	* digraph.cc (selftest::test_shortest_paths): Update
	shortest_paths init for new param.  Add test of
	SPS_TO_GIVEN_TARGET.
	* shortest-paths.h (enum shortest_path_sense): New.
	(shortest_paths::shortest_paths): Add "sense" param.
	Update for renamings.  Generalize to use "sense" param.
	(shortest_paths::get_shortest_path): Rename param.
	(shortest_paths::m_sense): New field.
	(shortest_paths::m_prev): Rename...
	(shortest_paths::m_best_edge): ...to this.
	(shortest_paths::get_shortest_path): Update for renamings.
	Conditionalize flipping of path on sense of traversal.