Further improvements to array comma linter rule

Summary:
Improve the ArcanistXHPASTLinter::LINT_ARRAY_SEPARATOR rule to be able to autofix the following code:

array(
  1,
  2,
  3, /* comment */ );

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12308