[ASan] Introduce SetCanPoisonMemory() function.

SetCanPoisonMemory()/CanPoisonMemory() functions are now used
instead of "poison_heap" flag to determine if ASan is allowed
to poison the shadow memory. This allows to hot-patch this
value in runtime (e.g. during ASan activation) without introducing
a data race.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224395 91177308-0d34-0410-b5e6-96231b3b80d8