Fix LINT_ARRAY_SEPARATOR for nested arrays

Summary: The LINT_ARRAY_SEPARATOR rule was added to ArcanistXHPASTLinter in D10535. Unfortunately, it does not correctly handle nested arrays correctly. This patch fixes this. Depends on D10554.

Test Plan: Added a test case.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10558