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