Fix a null pointer dereference when copying a null polymorphic pointer.

This bug only bit the C++98 build bots because all of the actual uses
really do move. ;] But not *quite* ready to do the whole C++11 switch
yet, so clean it up. Also add a unit test that catches this immediately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194548 91177308-0d34-0410-b5e6-96231b3b80d8