Make sure all the threads get a chance to compute their StopInfo's before we start running
ShouldStop on the threads, which might destroy information needed to correctly compute another 
thread's StopInfo.

<rdar://problem/13664026>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@179641 91177308-0d34-0410-b5e6-96231b3b80d8