This implements the proposal at
https://github.com/tc39/proposal-template-literal-revision
staged behind a flag --harmony-template-escapes. The proposal allows
invalid octal, unicode, and hexadecimal escape sequences to appear in
tagged template literals, instead of being a syntax error. These have
a 'cooked' value of 'undefined', but are still accessible through the
'raw' property.

BUG=v8:5546

Review-Url: https://codereview.chromium.org/2665513002
Cr-Commit-Position: refs/heads/master@{#43384}