Planner and ORCA translator both implemented the same logic, to assign
external table URIs to segments. But I spotted one case where the logic
differed:

CREATE EXTERNAL TABLE exttab_with_on_master( i int, j text )
LOCATION ('file://@hostname@@abs_srcdir@/data/exttab_few_errors.data') ON MASTER FORMAT 'TEXT' (DELIMITER '|');

SELECT * FROM exttab_with_on_master;
ERROR:  'ON MASTER' is not supported by this protocol yet.

With ORCA you got a less user-friendly error:

set optimizer=on;
set optimizer_enable_master_only_queries = on;
postgres=# explain SELECT * FROM exttab_with_on_master;
ERROR:  External scan error: Could not assign a segment database for external file (CTranslatorDXLToPlStmt.cpp:472)

The immediate cause of that was that commit fcf82234 didn't remember to
modify the ORCA translator's copy of the same logic. But really, it's silly
and error-prone to duplicate the code, so modify ORCA to use the same code
that the planner does.