It actually /does/ make sense to enforce the usable-from-inline rules
on dynamic declarations, but that would break source compatibility at
this point. Just allow '@usableFromInline' to be written on 'dynamic'
declarations, so that it'll be compatible with Swift 5.
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.
