Make sure the CallFunction Thread plans don't try to do DoTakedown if their thread
has gone  away by the time they get around to doing it.

<rdar://problem/15245544>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192987 91177308-0d34-0410-b5e6-96231b3b80d8