Implement instantiation of the declarations of member function

templates within class templates, producing a member function template
of a class template specialization. If you can parse that, I'm
sorry. Example:

  template<typename T>
  struct X {
    template<typename U> void f(T, U);
  };

When we instantiate X<int>, we now instantiate the declaration
X<int>::f, which looks like this:

  template<typename U> void X<int>::f(int, U);

The path this takes through
TemplateDeclInstantiator::VisitCXXMethodDecl is convoluted and
ugly, but I don't know how to improve it yet. I'm resting my hopes on
the multi-level substitution required to instantiate definitions of
nested templates, which may simplify this code as well.

More testing to come...

llvm-svn: 80252