BumpPtrAllocator::Reset should also poison the first slab which doesn't get deallocated.

When asan is enabled, we poison slabs as we allocate them, and only unpoison the pieces
we need from the slab.

However, in Reset, we were failing to reset the state of the slab back to being poisoned.

Patch by b17 c0de.

llvm-svn: 257388