Add a mutex lock to StackFrame to protect race conditions for
updating its ivars.  We've had a lot of crash reports and careful
analysis shows that we've got multiple threads operating on the
same StackFrame objects, changing their m_sc and m_flags ivars.
<rdar://problem/18406111> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218845 91177308-0d34-0410-b5e6-96231b3b80d8