Make the stop-on-sharedlibrary-events setting work.

We weren't taking into account the "m_should_stop" setting that the
synchronous breakpoint callback had already set when we did PerformAction
in the StopInfoBreakpoint.  So we didn't obey its instructions when it
told us to stop.  Fixed that and added some tests both for when we
just have the setting, and when we have the setting AND other breakpoints
at the shared library load notification breakpoint address.

Differential Revision: https://reviews.llvm.org/D98914