This adds proper poisoning/unpoisoning to segments put into the segment
pool of an accounting allocator, and also marks a segment uninitialized
when returning it from the pool. This gives ASan a better chance at
catching use-after-free and others.

Drive-by: Fix type check in ASAN_POISON_MEMORY_REGION

R=mstarzinger@chromium.org

Change-Id: Iadbdd7c0a0c80da8e7b9bb8f3399209715436073
Reviewed-on: https://chromium-review.googlesource.com/c/1489086
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59932}