This change moves the definitions of the bitfield flags used by Symbol
and Map to Torque. Symbol could directly follow the pattern established
by SharedFunctionInfo, but Map required some other changes:
- Until now, Torque bitfield definitions have required unsigned types. I
  thought that this would be the least-surprising behavior, since we
  never sign-extend when decoding bitfield values. However, I believe
  that the amount of churn involved in making ElementsKind be unsigned
  outweighs the benefit we were getting from this restriction (and
  similar difficulties are likely to arise in converting other bitfield
  structs to Torque), so this CL updates Torque to allow signed bitfield
  values.
- If we try to make Map extend from all of the generated classes that
  define its flags, we end up with class sizing problems because some
  compilers only apply empty base class optimization to the first in a
  row of empty base classes. We could work around this issue by
  generating macros instead of classes, but I took this as an
  opportunity for a minor clean-up instead: rather than having bitfield
  definitions for several different bitfield structs all jumbled
  together in Map, they can be split up. I think this makes the code a
  little easier to follow, but if others disagree I'm happy to implement
  macro generation instead.

Change-Id: Ibf339b0be97f72d740bf1daa8300b471912faeba
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1988934Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#65701}