Double initialization may cause the ExternalPointerTable compaction
algorithm to behave non-optimally: Consider the case of an Entry E1 that
is owned by a HeapObject O and is marked for evacuation during GC
marking. In that case, a new entry E2 is allocated for it, and during
sweeping, E1 will be evacuated into E2 and the Handle in O updated to
point to E2. However, if a new entry E3 for O is allocated before
sweeping, then during sweeping E3 (instead of E1) will be moved into E2.
This may then violate the invariant that the compaction algorithms
always evacuates an entry out of the evacuation area.

This CL therefore forbids double initializaiton of external pointer
slots and adds DCHECKs to attempt to catch these in debug builds.

Bug: v8:10391
Change-Id: I128dc930e8b3f863dab18ba648f34d68d8cb276b
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/+/3856563Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82729}