PR14742 - more complete dwz / DW_TAG_partial_unit support
Don't use DW_TAG_partial_units directly in dwflpp::iterate_over_cus
and dwarf_var_expanding_visitor::getcuscope. Descent into imported_units
in has_only_decl_members, dwflpp::iterate_over_globals,
dwflpp::iterate_over_types, dwflpp::iterate_over_labels
dwflpp::inner_die_containing_pc, dwflpp::print_locals,
dwflpp::print_members, dwflpp::find_struct_member and
dwarf_pretty_print::recurse_struct_members.