Change the looping stack detection code
In some special case (e.g. signal handlers, hand written assembly) it is
valid to have 2 stack frame with the same CFA value. This CL change the
looping stack detection code to report a loop only if at least 3
consecutive frames have the same CFA.
Note: I would prefer to get rid of this looping stack detection because the implementation is still a bit flaky and rely on the logic in UnwindLLDB::GetOneMoreFrame to stop the infinite loop in case of a looping stack, but I don't know how big the user impact will be in the cases where the unwind go wrong and start looping