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

unittests/ADT/DenseMapTest.cpp

@@ -46,7 +46,6 @@ public:
   CtorTester(const CtorTester &Arg) : Value(Arg.Value) {
     EXPECT_TRUE(Constructed.insert(this).second);
   }
-  CtorTester &operator=(const CtorTester &) = default;
   ~CtorTester() {
     EXPECT_EQ(1u, Constructed.erase(this));
   }