Reworked the way Process::RunThreadPlan and the ThreadPlanCallFunction interoperate to fix problems where
hitting auto-continue signals while running a thread plan would cause us to lose control of the debug 
session.

<rdar://problem/12993641>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@174793 91177308-0d34-0410-b5e6-96231b3b80d8