[tsan] Stop extending the block’s lifetime in dispatch_group_async

The dispatch_group_async interceptor actually extends the lifetime of the executed block. This means the destructor of the block (and captured variables) is called *after* dispatch_group_leave, which changes the semantics of dispatch_group_async.  This patch fixes that.

Differential Revision: http://reviews.llvm.org/D21816

llvm-svn: 274117