Make graphds_scc pass the node order back to callers

Make graphds_scc pass the node order back to callers

As a side-effect, graphds_scc constructs a vector in which all
nodes in an SCC are listed consecutively.  This can be useful
information, so that the patch adds an optional pass-back parameter
for it.  The interface is similar to the one for graphds_dfs.

gcc/
	* graphds.cc (graphds_scc): Add a pass-back parameter for the
	final node order.
	* graphds.h (graphds_scc): Update prototype accordingly.