Fix inline stepping test case on Linux because Thread::ThreadStoppedForAReason ignored virtual steps.
- add IsVirtualStep() virtual function to ThreadPlan, and implement it for
  ThreadPlanStepInRange
- make GetPrivateStopReason query the current thread plan for a virtual stop to
  decide if the current stop reason needs to be preserved
- remove extra check for an existing process in GetPrivateStopReason



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@181795 91177308-0d34-0410-b5e6-96231b3b80d8