pgsql: Fix alias-for-target-table-of-UPDATE-or-DELETE patch so that

Log Message:
-----------
Fix alias-for-target-table-of-UPDATE-or-DELETE patch so that alias can
be any ColId other than 'SET', rather than only IDENT as originally.
Per discussion.

Modified Files:
--------------
pgsql/src/backend/parser:
gram.y (r2.523 -> r2.524)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.523&r2=2.524)