If we stopped but no threads had a reason for stopping, we should tell the user about it rather than continuing.

<rdar://problem/13273125> Astris thread status replies for single-core device confuse lldb; lldb resumes execution on attaching

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@177670 91177308-0d34-0410-b5e6-96231b3b80d8