Allows users to specialize polymorphic_type_hook with std::enable_if.

Currently user specializations of the form

template <typename itype> struct polymorphic_type_hook<itype, std::enable_if_t<...>> { ... };

will fail if itype is also polymorphic, because the existing specialization will also
be enabled, which leads to 2 equally viable candidates. With this change, user provided
specializations have higher priority than the built in specialization for polymorphic types.