A CagedPointer is guaranteed to point into the Virtual Memory Cage and
will for example be used for ArrayBuffer backing stores when the heap
sandbox is enabled. In the current implementation, CagedPointers are
stored as offsets from the cage base, shifted to the left. Because the
cage base address is usually available in a register, accessing a
CagedPointer is very efficient, requiring only an additional shift and
add operation.

Bug: chromium:1218005
Change-Id: Ifc8c088e3862400672051a8c52840514dee2911f
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3123417Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77614}