[CallSite] Make construction from Value* (or Instruction*) explicit.

CallSite roughly behaves as a common base CallInst and InvokeInst. Bring
the behavior closer to that model by making upcasts explicit. Downcasts
remain implicit and work as before.

Following dyn_cast as a mental model checking whether a Value *V isa
CallSite now looks like this: 
  if (auto CS = CallSite(V)) // think dyn_cast
instead of:
  if (CallSite CS = V)

This is an extra token but I think it is slightly clearer. Making the
ctor explicit has the advantage of not accidentally creating nullptr
CallSites, e.g. when you pass a Value * to a function taking a CallSite
argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234601 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/AliasAnalysisEvaluator.cpp

@@ -158,7 +158,7 @@ bool AAEval::runOnFunction(Function &F) {
     if (EvalAAMD && isa<StoreInst>(&*I))
       Stores.insert(&*I);
     Instruction &Inst = *I;
-    if (CallSite CS = cast<Value>(&Inst)) {
+    if (auto CS = CallSite(&Inst)) {
       Value *Callee = CS.getCalledValue();
       // Skip actual functions for direct function calls.
       if (!isa<Function>(Callee) && isInterestingPointer(Callee))