Fix `LINT_ARRAY_SEPARATOR` for heredocs

Summary: It is not correct to add a trailing comma if the `n_ARRAY_VALUE` node is a heredoc string.

Test Plan: Added a test case.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10569