After we've completed a full backtrace, we'll have one frame which
is "invalid" -- it is past the end of the stack trace.  Add a new
method IsCompletedStackWalk() so we can tell if an invalid stack
frame is from a complete backtrace or if it might be worth re-trying
the last unwind with a different method.

This fixes the unwinder problems Ryan Brown was having with go
programs.  The unwinder can (under the right circumstances) still
destructively replace unwind plans permanently - I'll work on
that in a different patch.

<rdar://problem/18683658>