My original fix only addressed the issue for when the property was exactly internal, so
we would still run into problems with keypaths and `private(set)` when `-enable-testing`
is on, or when referring to `public` properties with private setters from the same module.
This generalizes the rule, so that the setter entry point for any property with
at least internal visibility also has at least internal visibility, even if the setter
is semantically less visible. Fixes rdar://78523318.
