This enables defining abstract type subtypes of classes
with constexpr version, which in turn is useful to model
custom C++ subclasses.

Bug: v8:7793
Change-Id: I07dcb62121afdddfbe2c78ecc870afcb11800c19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2412180
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69982}