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