The effective access of an overridden declaration is subject to
escalation by -enable-testing. When this is flag is enabled, an
interface containing an internal-overriding-public declaration will
still print `override`. This is because the effective access of the base
of the override is formally internal but effectively public.
Instead, use the formal access scope of the overridden declaration to
compute its access relative to the override. While I'm here, catch the
case where the base declaration is `@usableFromInline` and therefore
*will* be printed in the interface. We treat these declarations as
effectively public for the purpose of printing `override`.
Resolves rdar://64969741
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
