There is no particular reason that PropertyDescriptorObject should be a
subclass of FixedArray. By using a separate struct type, we get better
generated accessor functions, automatic verification, and runtime type
info, plus we save four bytes per instance.

Change-Id: If076782832aa9398806794e4ee6d019aea2f92b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1999463Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#65756}