When closing untagged template string literals, create a single n-ary
addition operation, instead of a tree of binary operations.

As a clean-up, this also entirely removes the "second" field from n-ary
operations. This was proving to be too confusing an API when building
an n-ary operation incrementally from a single expression (rather than
converting a binary operation).

Bug: v8:6964
Change-Id: I8f2a395d413cf345bab0a1a347b47f412cde83b1
Reviewed-on: https://chromium-review.googlesource.com/739821Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49054}