pgsql: Fix bogus grammar for a CREATE CONSTRAINT TRIGGER error

Fix bogus grammar for a CREATE CONSTRAINT TRIGGER error

If certain constraint characteristic clauses (NO INHERIT, NOT VALID, NOT
ENFORCED) are given to CREATE CONSTRAINT TRIGGER, the resulting error
message is
ERROR: TRIGGER constraints cannot be marked NO INHERIT
which is a bit silly, because these aren't "constraints of type
TRIGGER". Hardcode a better error message to prevent it. This is a
cosmetic fix for quite a fringe problem with no known complaints from
users, so no backpatch.

While at it, silently accept ENFORCED if given.

Author: Amul Sul <sulamul(at)gmail(dot)com>
Reviewed-by: jian he <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://postgr.es/m/CAAJ_b97hd-jMTS7AjgU6TDBCzDx_KyuKxG+K-DtYmOieg+giyQ@mail.gmail.com
Discussion: https://postgr.es/m/CACJufxHSp2puxP=q8ZtUGL1F+heapnzqFBZy5ZNGUjUgwjBqTQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/87251e114967d668c8f90ed9fb8c8a8834c2d288

Modified Files
--------------
doc/src/sgml/ref/create_trigger.sgml | 9 ++++++++-
src/backend/parser/gram.y | 22 +++++++++++++++++++++-
src/test/regress/expected/triggers.out | 23 ++++++++++++++++++++++-
src/test/regress/sql/triggers.sql | 15 ++++++++++++++-
4 files changed, 65 insertions(+), 4 deletions(-)