Stop-hooks weren't getting called on step-out. Fix that.

There was a little bit of logic in the StopInfoBreakpoint::PerformAction
that would null out the StopInfo once we had a completed plan so that the
next call to GetStopInfo would replace it with the StopInfoThreadPlan.

But the stop-hooks check for whether a thread stopped for a reason didn't
trigger this conversion.  So I added an API to do that directly, and then
called it where before we just reset the StopInfo.

<rdar://problem/54270767>

Differential Revision: https://reviews.llvm.org/D66241

llvm-svn: 369052