Incorrectly resolved base template specialisations

[ddobrev]

If a class derives from a template specialisation, our AST reports its base as a the template class and not as the specialisation. The following example in particular causes a crash:

template <int N, int... I>
struct build_index_impl : build_index_impl<N - 1, N - 1, I...> {};

[ddobrev]

Clang seems not to report a declaration for "build_index_impl<N - 1, N - 1, I...>" - just a type. Since we mirror Clang, we should have no (null) declaration too for BaseClassSpecifier.Class.