Summary of changes:

- GC visitors no longer rely on superclass visitors, but instead visit
  everything themselves. This enables generating better code.
- Try to match simple body descriptors to reduce the amount of generated
  code.
- Turn SizeFor(instance) into an AllocatedSize() method.
- Remove the special handling of resizable object sizes from Torque
  and instead overwrite AllocatedSize in classes that need special
  handling in C++.
- Split the visitor id lists depending on whether the class has pointer
  fields.
- Turn Torque-generated body descriptors into an .inc file to
  simplify includes.
- Fix generated size functions to properly align the size.
- Generate GC visitors (and C++ class definitions) for all string
  classes and FixedArray, WeakFixedArray, and WeakArrayList.
- Store generated instance types in Torque class types. This is only
  used to determine if a type has a single instance type in this CL.

Bug: v8:7793
Change-Id: I4d362e96b047c305bd6d065247734957b8958c42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2110014
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67542}