Fix PHP compatibility linter after improvements to parsing of use statements
Summary: Fix the PHPCompatibilityXHPASTLinterRule after changes to the way in which #xhpast parses use statements. Depends on D14518.
Test Plan: Unit tests
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14519