Improve array comma rule

Summary: Improve ArcanistXHPASTLinter::LINT_ARRAY_SEPARATOR in handling multi-line arrays, see D12280 and D12281 for example. Depends on D12295.

Test Plan: Updated unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12296