[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.

llvm-svn: 234601
diff --git a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
index 2ecf7b2..de323b9 100644
--- a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
+++ b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
@@ -482,7 +482,7 @@
       return Result;
     }
 
-    if (ImmutableCallSite CS = V) {
+    if (auto CS = ImmutableCallSite(V)) {
       const Function *F = CS.getCalledFunction();
       if (F) {
         // Used in a direct call.