rand() is not a constant expr

Also fixes a bug in Expr::DebugString()

Change-Id: I32b53072755781d0858481187864d2319b9ae1cb
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5400
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins
(cherry picked from commit 6de9fab17a5032dd7c9d1ef6b8071703c67d223f)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5425
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>