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

Make the code in TriggerDescriptor.getActionSPS() reusable for
TriggerDescriptor.getWhenClauseSPS() so that the fixes for DERBY-4874
and Cloudscape bug 4821 also get applied to the WHEN clause.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1531226 13f79535-47bb-0310-9956-ffa450edef68