frames might go away (the object itself, not the actual logical frame) when
we are single stepping due to the way we currently sometimes end up flushing
frames when stepping in/out/over. They later will come back to life 
represented by another object yet they have the same StackID. Now when you get
a lldb::SBFrame object, it will track the frame it is initialized with until 
the thread goes away or the StackID no longer exists in the stack for the 
thread it was created on. It uses a weak_ptr to both the frame and thread and
also stores the StackID. These three items allow us to determine when the
stack frame object has gone away (the weak_ptr will be NULL) and allows us to
find the correct frame again. In our test suite we had such cases where we
were just getting lucky when something like this happened:

1 - stop at breakpoint
2 - get first frame in thread where we stopped
3 - run an expression that causes the program to JIT and run code
4 - run more expressions on the frame from step 2 which was very very luckily
    still around inside a shared pointer, yet, not part of the current 
    thread (a new stack frame object had appeared with the same stack ID and
    depth). 
    
We now avoid all such issues and properly keep up to date, or we start 
returning errors when the frame doesn't exist and always responds with
invalid answers.

Also fixed the UserSettingsController  (not going to rewrite this just yet)
so that it doesn't crash on shutdown. Using weak_ptr's came in real handy to
track when the master controller has already gone away and this allowed me to
pull out the previous NotifyOwnerIsShuttingDown() patch as it is no longer 
needed.

llvm-svn: 149231