RFR 8188098: NPE in SimpleTreeVisitorES6 visitor when parsing a tagged template literal

[Sundararajan Athijegannathan]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8188098
Webrev: http://cr.openjdk.java.net/~sundar/8188098/webrev.00/

Thanks,
-Sundar