Removes the AstString super-class and makes the two sub-classes stand-alone.
That also removes the slightly awkward implementation of a super-class with
dynamically dispatched behaviour, except it was manually implemented because
virtual function calls were considered expensive.

BUG=v8:6902

Change-Id: I079a2372557aa17ee839efcaddb9b7fe86e38b4d
Reviewed-on: https://chromium-review.googlesource.com/458428
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44104}