The util/visit_helper.h defines a useful `Overloaded` struct for use with `std::visit` or `stdx::visit` of a variant.

https://github.com/10gen/mongo/blame/master/src/mongo/util/visit_helper.h#L33

We should eliminate the "helper" from its name and not give it a private namespace containing only a single function. There's no technical reason to give it its own namespace. It seems to be just more typing.

As it's in util/ it's a kind of precedent setter so I would like to take the namespace out of the picture as we don't do this for other util/ offerings typically.