[asan] Re-poison all redzones on activation.

When running with start_deactivated=1 in ASAN_OPTIONS, heap redzones
are not poisoned until the first instrumented module is loaded. This
can cause false negatives even on memory allocated after activation,
because redzones are normally poisoned only once when a new allocator
region is mapped.

This change attempts to fix it by iterating over all existing
allocator chunks and poisoning their redzones.

llvm-svn: 281364