Add the critically missing 'clone' method. =]

Clang managed to never instantiate the copy constructor. Added tests to
ensure this path is tested.

We could still use tests for the polymorphic nature. Those coming up
next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194317 91177308-0d34-0410-b5e6-96231b3b80d8