swift234

In the new call graph, find the SCCs in bottom-up order.

With this change we generate the SCCs in the call graph (for the kinds
of calls we currently recognize) in bottom-up, which we can then
iterate over directly or in reverse for invocation- and
reverse-invocation-order traversal of the call graph.

(This still isn't hooked up - but will be after some dumping routines
and verification of the output).

Swift SVN r21552