[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.

llvm-svn: 224395