... for AbstractValue and AbstractControl. This allows to access the
fields directly instead of via {interface_data}, and hence makes the code
more readable. Also, it makes AbstractValue and AbstractControl
non-templates. They are also renamed to ValueBase and ControlBase.
Unfortunately, it requires the introduction of new templates
ValueWithNamedConstructors and ControlWithNamedConstructors, in order to
provide correctly typed named constructors (we cannot define implicit
conversion from {Value,Control}Base to a subtype because of our style
guide, checked by a presubmit check).

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Icb7796d040afbf92309333a03d4286fc782f8d1b
Reviewed-on: https://chromium-review.googlesource.com/643392Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47747}