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