DeltaAlgorithm: Provide protected default copy ctor for use by test derived class.

Without this, use of this copy ctor is deprecated in C++11 due to the
presence of a user-declared dtor.

Marking the class final is just a little extra security that there are
no further derived classes that may then end up using the intermediate
base class's copy assignment operator and cause slicing to occur.

I didn't bother marking the other (non-test) base class final, since it
has reference members so it won't have any implicit assignment operators
anyway. Open to ideas on that, though.

We probably want a warning about use of a slicing assignment operator,
then I wouldn't worry so much about marking the class as final.

llvm-svn: 231114