Fix a cornercase in breakpoint reporting
This resolves a similar problem as D16720 (which handled the case when we single-step onto a
breakpoint), but this one deals with involutary stops: when we stop a thread (e.g. because
another thread has hit a breakpont and we are doing a full stop), we can end up stopping it right
before it executes a breakpoint instruction. In this case, the stop reason will be empty, but we
will still step over the breakpoint when do the next resume, thereby missing a breakpoint hit.
I have observed this happening in TestConcurrentEvents, but I have no idea how to reproduce this
behavior more reliably.
Differential Revision: http://reviews.llvm.org/D18692