When the LeakSanitizer (LSan) runs, it scans all reachable memory
looking for pointers to other (live) objects, then reports all objects
that are still allocated but not reachable as leaked.
When the external pointer table is used, the pointers stored in it do
unfortunately not look like pointers to LSan as they will have some of
the top bits set. As such, LSan ignores them and may afterwards
incorrectly report some referenced objects as leaked.
To fix this, we now use a "shadow table" when LSan is active which
contains the raw pointer for every (tagged) pointer stored in the real
table. LSan can then scan this table and find all references.

Bug: v8:10391
Change-Id: If0c8b042fdd775ac3c8025d5688e62df37532ec3
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/+/3779915
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81972}