If ThreadPlanCallFunction hasn't set its notion of the "real stop info" yet, just return the current PrivateStopInfo.

Also renamed a few more places where we were using StopReason in functions that were returning StopInfo's.

<rdar://problem/14042692>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183177 91177308-0d34-0410-b5e6-96231b3b80d8