Refactor getting children name, value and type access

    Refactor getting children name, value and type access
        for varobjs in C++.
        * varobj.c (get_type_deref): Remove.
        (adjust_value_for_child_access): New.
        (c_number_of_children): Use the above.
        (c_describe_child): Likewise.
        (enum accessibility): New.
        (match_accessibility): New function.
        (cplus_describe_child): New function.
        (cplus_name_of_child, cplus_value_of_child)
        (cplus_type_of_child): Reimplement in terms
        of cplus_describe_child.
        (cplus_number_of_children): Use
        adjust_value_for_child_access.