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@gmail.com>
Reviewed-by: jian he <jian.universality@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.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