Only allow BindingIdentifier in BindingRestPattern and 
ValidReferenceExpression in AssignmentRestPattern.

Also updated to a better, actionable error message.

Bug: v8:6500, v8:6513
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ifaba2f85c20bc20e263267e8c76d50a27075b87d
Reviewed-on: https://chromium-review.googlesource.com/550559
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46653}