... and mark it as never-serialized wrt turbofan serialization.

Until this CL, the JSRegExp type was used as both for plain
user-visible regexp objects, and for internal regexp boilerplate
descriptions. Boilerplates are special: they are never exposed to the
user, they are only referenced from the feedback vector, they are
immutable.

To clarify this distinction, this CL introduces a dedicated struct
type RegExpBoilerplateDescription to hold the regexp boilerplate
description.

This makes Turbofan serialization simpler: boilerplates can be
accessed through direct reads since they are immutable. TF has no
special requirements on JSRegExp objects (it never reads into these
objects) and thus serializing only the references as a JSObjectRef is
fine.

Bug: v8:7790
Change-Id: I33b337fcfcf861a02bc6be6d0c6311d07cf05718
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2656257Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72522}