DERBY-534: Support use of the WHEN clause in CREATE TRIGGER statements

Reject references to generated columns in the NEW transition variables
of BEFORE triggers, as required by the SQL standard. See also
DERBY-3948.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1527489 13f79535-47bb-0310-9956-ffa450edef68