Refactor SpacePolicy on a non-templated class to avoid the situation
of having MakeGarbageCollectedTraitBase<T>::SpacePolicy<U> refer to
different T and U which make it hard for the compiler to alias
anything.

Bug: chromium:1056170
Change-Id: I78eb0362d43403ad2712bcb65746eeb9f6ad44fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2769338Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73494}