Fix Breakpoint::RemoveInvalidLocations to fix the exec testcase.

RemoveInvalidLocations was clearing out the m_locations in the
breakpoint by hand, and it wasn't also clearing the locations from
the address->location map, which confused us when we went to update
breakpoint locations.  

I also made Breakpoint::ModulesChanged check the Location's Section
to make sure it hadn't been deleted.  This shouldn't strictly be necessary,
but if the DynamicLoaderPlugin doesn't do it's job right (I'm looking at
you new Darwin DynamicLoader...) then it can end up leaving stale locations
on rerun.  It doesn't hurt to clean them up here as a backstop.

<rdar://problem/36134350>

llvm-svn: 322348