This change adds a new code generator, which supports a subset of the
instructions supported by the existing CSAGenerator, and instead of
generating CSA it generates runtime C++ code. The new generator is used
to generate a set of Torque macros that return slices to indexed fields.
These new macros should be sufficient to eventually support
Torque-generated field accessors, BodyDescriptors, verifier functions,
and postmortem field inspection in debug_helper.

Bug: v8:7793
Change-Id: Ife2d25cfd55a08238c625a8b04aca3ff2a0f4c63
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2429566Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#70313}