Special-case System.Enum as a generic constraint

[ds5678]

Previously:

public class AdapterView<T, VH, T_AXIS_LIST> : MonoBehaviour
    where VH : ViewHolder<T>
    where T_AXIS_LIST : IL2CppSystem.Enum
{
}

Now:

public class AdapterView<T, VH, T_AXIS_LIST> : MonoBehaviour
    where VH : ViewHolder<T>
    where T_AXIS_LIST : System.Enum
{
}