Add a lock in the UnwindTable class so two Targets won't try
to modify the same UnwindTable object simultaneously.  Fix
HistoryThread and HistoryUnwind's mutex lock acqusition to
retain the lock for the duration of the operation instead of
releasing the temporary immediately.
<rdar://problem/17055023>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211241 91177308-0d34-0410-b5e6-96231b3b80d8