swift234

[callgraph] Add basic verification of the callgraph.

This is currently disabled by default since this verification trips on
ToT when I checked a few days ago.

The current checks that are performed are:

1. For every (SILFunction, CallGraphNode) pair FuncPair in the SILFunction to
   CallGraphNode map check that:
   a. FuncPair.first is a SILFunction in the current module.
   b. FuncPair.first is the SILFunction inside the CallGraphNode FuncPair.second.
   c. All CallGraphEdges mapped to FuncPair.second have ApplyInsts which are in
      the SILFunction FuncPair.first.

2. For every pair (ApplyInst, CallGraphEdge) ApplyPair in the Apply to
   CallGraphEdge map, check that:
   a. ApplyPair.second.getCallSite() == ApplyPair.first.
   b. ApplyPair.first->getFunction() is in the SILFunction to
      CallGraphNode map and the CallGraphEdge for ApplyPair is one of
      CallSiteEdges in the mapped to CallGraphNode.

<rdar://problem/19944399>

Swift SVN r25520