Rework the implementation of non-external Torque classes to use
Struct machinery rather than FixedArray machinery. This allows
Torque-only defined 'internal' classes to the automatically generate
class verifiers and printers.

As part of this change, generate C++ boilerplate accessors for
internal Torque classes, since this is a pre-requisite for the
verifiers, printers and other Struct-based functionality.

Moreover, augment the header-generating functionality in Torque
to create separate header files for field offset definitions,
internal class C++ definitions and instance types.

Bug: v8:7793
Change-Id: I47d5f1570040c2b44d378f23b6cf95d3d132dacc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1607645
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62317}