This is not yet hooked up. I want to add a dumper and some tests, and
need to implement code to do invocation- and reverse-invocation-ordering
over the SCCs of the call graph before it can fully replace the existing
code.
This call graph has edges for each call site, so it's really a
multigraph. Each call site tracks the set of functions that can be
called from that point. Currently that set is always a singleton since
we only handle applies of direct function_refs (like the existing call
graph). In time it will be expanded to handle other types of function
application.
Swift SVN r21407
