[tsan] Update dispatch_group support to avoid using a disposed group object

We're using the dispatch group itself to synchronize (to call Release() and Acquire() on it), but in dispatch group notifications, the group can already be disposed/deallocated. This causes a later assertion failure at `DCHECK_EQ(*meta, 0);` in `MetaMap::AllocBlock` when the same memory is reused (note that the failure only happens in debug builds).

Fixing this by retaining the group and releasing it in the notification. Adding a stress test case that reproduces this.

Differential Revision: http://reviews.llvm.org/D15380

llvm-svn: 255494