Implement instantiation of enums within class templates. This isn't

quite as great as it sounds, because, while we can refer to the
enumerator values outside the template, e.g.,

  adder<long, 3, 4>::value

we can't yet refer to them with dependent names, so no Fibonacci
(yet). 

InstantiateClassTemplateSpecialization is getting messy; next commit
will put it into a less-ugly state.

llvm-svn: 67092