Previously we made sure that inlinable function bodies only
reference @usableFromInline or public declarations, but we
also have to make sure that @usableFromInline declarations
only reference other declarations that are at least as
visible.
Otherwise, you could, for example, define a @usableFromInline
typealias which referenced an internal type, and then
reference the typealias from a @usableFromInline function body.
