for negation

Pull containsPatternVariable() up from Expression to Statement to
benefit code assist.

Change-Id: Ibcc07b03a242ac521f0a5a2d5ad4e46829679aaf
Signed-off-by: Jay Arthanareeswaran <jarthana@in.ibm.com>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/180451
Tested-by: JDT Bot <jdt-bot@eclipse.org>