[asan] Don't double poison secondary allocations

Sanitizers allocate shadow and memory as MAP_NORESERVE.

User memory can stay this way and do not increase RSS as long as we
don't store there.

The shadow unpoisoning also can avoid RSS increase for zeroed pages.
However as soon we poison the shadow, we need the page in RSS.

To avoid unnececary RSS increase we should not poison memory just before
unpoisoning them.

Depends on D153497.

Reviewed By: thurston

Differential Revision: https://reviews.llvm.org/D153500