Add a rule to the XHPAST linter to check for whitespace before a semicolon.

Summary: This rule is based on a rule from PHP_CodeSniffer.

Test Plan: Wrote and executed unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9195