Previously, each provider would be consulted until a matching parent is
found. This is incorrect and caused spurious warnings with --live
(jar + runtime inheritance), as unmatched classes in the jar would fall
through to the runtime inheritance provider.
Now once a provider returns non-null, no other providers are checked.
null indicates the provider knows nothing about the class and should
ask elsewhere, non-null indicates a definitive list of inheritance
(extends/implements list) classes. Should be slightly faster, too.
7ed216d944