DERBY-6690 ROW_NUMBER should not be allowed in generation clause

Patch *derby-6690* makes this illegal by adding a check in
TableElementList#bindAndValidateGeneration. I had to add an
acceptChildren method to GenerationClauseNode for the visitor to reach
down into the generation clause. Added a test to
OLAPTest#testBasicOperations for both CREATE TABLE and ALTER TABLE ADD
COLUMN.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1616365 13f79535-47bb-0310-9956-ffa450edef68