Also needed to fix up `TestUtil/UnresolvedCalls.qll` after a bad merge
conflict resolution. Since all calls are now DataFlowCall, and not JUST
the ones that can be resolved, we need to put in the restriction that
the callable can also be resolved.
This required a minor change in the type tracker implementation, but
apart from that no other changes appear to be needed. Seems to clean
up the test output quite a bit.