Minor refactors to improve readability and consistency between
FeedbackVectorSpec and FeedbackMetadata:

- Rename FeedbackVectorSpec::slots to slot_count.
- Rename FeedbackVectorSpec::closure_feedback_cells to
  create_closure_slot_count, likewise all related fields.
- Store FeedbackVectorSpec::slot_kinds_ as an array of
  FeedbackSlotKind.

Bug: v8:8888
Change-Id: I3a45177163d1484b1625de8dfba5c6c05cfc426d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2512908Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70943}