We probably don't want Any, that sounds like a footgun. And as said, this requires you to include buildExpression, which was optional before.
That's why I think post-buildExpression Void expressions could be ignored, it could work with or without buildExpression, and I don't expect any builders to lose its functionality. If anything, they can opt-in to include Void by adding:
func buildExpression(_: ()) -> Empty
Which is ceremonious, but seems to be a much rarer case.