Fix the part of stepping over watchpoints that lldb can fix

[jimingham]

There are two bugs with stepping over watchpoint hits in lldb. One is an lldb bug - the ThreadPlanStepOverWatchpoint needs to be registered as a "Controlling Plan" or it won't trigger a public stop when it completes. The other is a kernel issue where if an instruction single step triggers the wp, we only get the single step completion exception, not the watchpoint hit.

But since wp triggering instructions are seldom branch instructions, and lldb runs from branch to branch using breakpoints, with this fix you will only miss the watchpoint hit if you are unlucky enough for the faulting instruction to be the first one in the source line, or the first one in the next source line.

So it's worth having this fix even if we haven't gotten the kernel fix yet.

[jimingham]

@swift-ci please test