Make the mutex ivar in Unwind recursive so we don't have a thread
deadlocking when we have the base Unwind class and the HistoryUnwind
subclass both trying to acquire the lock on the same thread to clear
their respective ivar state.
<rdar://problem/18986350> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222221 91177308-0d34-0410-b5e6-96231b3b80d8