Reparenting: disallow overriding by inheritees
If an inheriting protocol IP declares a requirement that
exactly matches one in the reparentable protocol P, we cannot
consider it to be an override. They need to have independent
witness table entries in both IP and P.
Otherwise, reparenting IP would cause those overriding decls
to have witness table entries moved from IP to P, causing an
ABI break.
resolves rdar://171928937
