Rather than an Object array, use a Tagged_t array to store the
elements of the off-heap string table. This matches the old on-heap
string table's behaviour, and recovers memory regressions from that
work.

To be able to do this, this also introduces a new slot type,
OffHeapObjectSlot. This is because CompressedObjectSlot assumes that
the slot is on-heap, and that it can mask the slot location to
recover the isolate root. OffHeapObjectSlot doesn't define an
operator*, and instead provides a `load(const Isolate*)` method.
The other slots also gain this method so that they can use it in
slot-templated functions. Also, the RootVisitor gains an
OffHeapObjectSlot overload, which is UNREACHABLE by default and only
needs to be defined by visitors that can access the string table.

As a drive-by, fix some non-atomic accesses to the off-heap string
table, also using the new slot.

Bug: chromium:1109553
Bug: chromium:1115116
Bug: chromium:1115559
Bug: chromium:1115683
Change-Id: I819ed7bf820e9ef98ad5d5f9d0d592efbb6f5aa6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2352489
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69381}