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