ES6 template literals: Fix issue with template after rbrace

If we hade }` the right brace was always treated as part of the
template literal. We should only treat the right brace as part of
the literal when we continue to parse the template literal after a
placeholder.

BUG=v8:3734
LOG=Y

Review URL: https://codereview.chromium.org/778813003

Cr-Commit-Position: refs/heads/master@{#25661}