ScopeInfo has a vestigial 'length' field from when it used to be a
FixedArray. This change removes that field, which saves some memory.

More specifically:

- Make ScopeInfo inherit from HeapObject, not FixedArrayBase which
  supplied the 'length' field.
- Privatize the FixedArray-style functions that provide access to
  ScopeInfo fields by index, and move them from scope-info-inl.h to
  scope-info.cc. Those functions are still used pretty heavily during
  initialization (ScopeInfo::Create, etc.), but at least we can avoid
  presenting them to the rest of the world.
- Change FactoryBase::NewScopeInfo to allocate the updated object shape.
  It maintains the existing behavior of filling the newly-allocated
  object with undefined, even though that's not a valid ScopeInfo and
  further initialization is required.
- Move part of AccessorAssembler::ScriptContextTableLookup into a new
  Torque macro, because it used to rely on casting ScopeInfo to
  FixedArrayBase.
- In V8HeapExplorer::AddEntry, don't claim that ScopeInfo objects are
  arrays. I think it makes more sense to list them under "(system)" in
  the dev tools, like most other V8 internal types.

Bug: v8:8952
Change-Id: I8278e3a90027d4409f0d268da0fe7080754c6b8c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2601880Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#72830}