Fixed an issue with the UnwindLLDB code where if there were inlined stack

frames, UnwindLLDB could create the incorrect RegisterContext for a given
stack frame because it was using the frame index (not the concrete frame
index). This was causing crashes when doing backtraces through the 
SBFrame::GetFP() because a NULL register context was being returned for the
deepest stack frame.

llvm-svn: 123052