Use PHP type hinting in ArcanistXHPASTLinter.

Summary: Explicitly specify the types of the function parameters. This change is basically the same as D8388.

Test Plan: arc unit.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9040