Add ArrayRef constructor from None, and do the cleanups that this constructor enables

Patch by Robert Wilhelm.

llvm-svn: 181138
diff --git a/llvm/lib/Transforms/Utils/ValueMapper.cpp b/llvm/lib/Transforms/Utils/ValueMapper.cpp
index b5941bd..544c5ee 100644
--- a/llvm/lib/Transforms/Utils/ValueMapper.cpp
+++ b/llvm/lib/Transforms/Utils/ValueMapper.cpp
@@ -57,7 +57,7 @@
       return VM[V] = const_cast<Value*>(V);
     
     // Create a dummy node in case we have a metadata cycle.
-    MDNode *Dummy = MDNode::getTemporary(V->getContext(), ArrayRef<Value*>());
+    MDNode *Dummy = MDNode::getTemporary(V->getContext(), None);
     VM[V] = Dummy;
     
     // Check all operands to see if any need to be remapped.