Revert "Remove the explicit SDNodeIterator::operator= in favor of the implicit default"

Accidentally committed a few more of these cleanup changes than
intended. Still breaking these out & tidying them up.

This reverts commit r231135.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231136 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-diff/DiffLog.h

@@ -40,8 +40,6 @@ namespace llvm {
   public:
     LogBuilder(Consumer &c, StringRef Format)
       : consumer(c), Format(Format) {}
-    // Relying on RVO, not actually copyable.
-    LogBuilder(const LogBuilder&);
 
     LogBuilder &operator<<(Value *V) {
       Arguments.push_back(V);