Make TupleDescInitBuiltinEntry throw error for unsupported types.

Previously, it would just pass back a partially-uninitialized tupdesc,
which doesn't seem like a safe or useful behavior.

Backpatch to v10 where this code came in.

Discussion: https://postgr.es/m/30830.1544384975@sss.pgh.pa.us

src/backend/access/common/tupdesc.c

@@ -643,6 +643,9 @@ TupleDescInitBuiltinEntry(TupleDesc desc,
 			att->attstorage = 'p';
 			att->attcollation = InvalidOid;
 			break;
+
+		default:
+			elog(ERROR, "unsupported type %u", oidtypeid);
 	}
 }