With caged heap enabled, we can halve Member<> by storing only the least
significant half. The base of the heap is stored in a thread local
variable. The feature has therefore an implication that only single heap
is allowed per thread.

The feature is gated by the new GN arg:
  cppgc_enable_pointer_compression.

Bug: chromium:1325007

Change-Id: Ic7f1ecb7b9ded57caad63d95bbc8e8ad6ad65031
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2739979Reviewed-by: Almothana Athamneh <almuthanna@chromium.org>
Commit-Queue: Almothana Athamneh <almuthanna@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80518}