Methods defined on `PrivateRelation` should return `T.self_type`

[rzane]

I wrote a compiler for Kaminari that returns the following:

sig { T.all(PrivateRelation, Kaminari::PageScopeMethods, Kaminari::ActiveRecordRelationMethods) }
def page(num = nil); end

This type is accurate, since .page uses .extending to add methods to the relation.

This works:

User.page(5).total_count

This doesn't work:

User.page(5).all.total_count
                 ^^^^^^^^^^^ Method `total_count` does not exist on `User::PrivateRelation`

That's because the return type of for .all is PrivateRelation. We could fix this problem by using T.self_type instead, which I believe more accurately captures the way ActiveRecord works.

The way that relevant compilers are currently implemented makes this a little tricky, since the methods that are being added to the PrivateRelation are also added to the model. The methods defined on the model should be PrivateRelation.