When we are printing Swift interface, we have to skip the override keyword
if the overriden decl is invisible from the interface. Otherwise, an error
will occur while building the Swift module because the overriding decl
doesn't override anything.
We couldn't skip every `override` keywords because they change the
ABI if the overriden decl is also publicly visible.
For public-override-internal case, having `override` doesn't have ABI
implication. Thus we can skip them.
rdar://58562780
