[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

lib/SILAnalysis/CallGraphAnalysis.cpp

@@ -276,3 +276,68 @@ void CallGraph::computeBottomUpFunctionOrder() {
     for (auto *Node : SCC->SCCNodes)
       BottomUpFunctionOrder.push_back(Node->getFunction());
 }
+
+void CallGraph::verify() const {
+#ifndef NDEBUG
+  // For every function in the module, add it to our SILFunction set.
+  llvm::DenseSet<SILFunction *> Functions;
+  for (auto &F : M)
+    Functions.insert(&F);
+
+  // 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 callee CallGraphEdges mapped to FuncPair.second have ApplyInsts
+  //       which are in the SILFunction FuncPair.first.
+  //
+  for (auto &P : FunctionToNodeMap) {
+    assert(Functions.count(P.first) && "Func in FunctionToNodeMap but not "
+                                       "in module!?");
+    assert(P.second->getFunction() == P.first &&
+           "Func mapped to node, but node has different Function inside?!");
+    for (CallGraphEdge *Edge : P.second->getCalleeEdges()) {
+      assert(Edge->getApply()->getFunction() == P.first &&
+             "ApplyInst in callee set that is not in the Callee function?!");
+    }
+  }
+
+  // For every pair (ApplyInst, CallGraphEdge) ApplyPair in the Apply to
+  // CallGraphEdge map, check that:
+  //
+  //    a. ApplyPair.second.getApply() == 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.
+  //
+  for (auto &P : ApplyToEdgeMap) {
+    assert(P.second->getApply() == P.first &&
+           "Apply mapped to CallSiteEdge but not vis-a-versa?!");
+    assert(Functions.count(P.first->getFunction()) &&
+           "Apply in func not in module?!");
+    CallGraphNode *Node = getCallGraphNode(P.first->getFunction());
+    assert(Node && "Apply without call graph node");
+
+    bool FoundEdge = false;
+    for (CallGraphEdge *Edge : Node->getCalleeEdges()) {
+      if (Edge == P.second) {
+        FoundEdge = true;
+        break;
+      }
+    }
+    assert(FoundEdge && "Failed to find Apply CallGraphEdge in Apply inst "
+                        "parent function's caller");
+  }
+#endif
+}
+
+void CallGraphAnalysis::verify() const {
+#ifndef NDEBUG
+  // If we don't have a callgraph, return.
+  if (!CG)
+    return;
+  CG->verify();
+#endif
+}