Technically, these operations belong in the ObjectiveC module, where NSObject
is defined. Keep them there. However, we need to build the mock ObjectiveC
overlay with `-disable-objc-attr-requires-foundation-module` now.
More specifically, don't try to emit a definition for them. Just fall
through to what we do for forward-declarations...which also needed some
fixing, to make sure we don't use a Swift typealias as its underlying
type but never import the underlying type.
https://bugs.swift.org/browse/SR-2352