Overview:
- Change basic type hierarchy to split Tagged into StrongTagged (= Object) and
  and WeakHeapObject. This enables to emit the right CSA types (Object, MaybeObject).
- The new Weak<T> type encodes a possibly cleared weak bit pattern that
  points to type T if it's not cleared.
- Make TNode<Object> a subtype of TNode<MaybeObject> so that the generated code
  compiles on the C++ side. Drive-by change: simplify a few CSA helpers by using
  MaybeObject as a common supertype of MaybeObject and Object.
- Port CreateObjectWithoutProperties and LoadMapPrototypeInfo.

Bug: v8:7793
Change-Id: I895a6501ce3e287ea8cf4065aaff3a5535245ab4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1889870Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64876}