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