Assert we don't create recursive DW_AT_{abstract_origin,specification}

Assert we don't create recursive DW_AT_{abstract_origin,specification}

We have a support case that shows GCC 7 sometimes creates
DW_TAG_label refering to itself via a DW_AT_abstract_origin
when using LTO.  This for example triggers the sanity check
added below during LTO bootstrap.

Making this check cover more than just DW_AT_abstract_origin
breaks bootstrap on trunk for

      /* GNU extension: Record what type our vtable lives in.  */
      if (TYPE_VFIELD (type))
        {
          tree vtype = DECL_FCONTEXT (TYPE_VFIELD (type));

          gen_type_die (vtype, context_die);
          add_AT_die_ref (type_die, DW_AT_containing_type,
                          lookup_type_die (vtype));

so the check is for now restricted to DW_AT_abstract_origin
and DW_AT_specification both of which we follow within get_AT.

	* dwarf2out.cc (add_AT_die_ref): Assert we do not add
	a self-ref DW_AT_abstract_origin or DW_AT_specification.