In the Active Record usage, a `BindParam` object always has an attribute
object as the value. A `BindParam` object is used to call `add_bind` to
generate placeholder if `prepared_statements: true`.

Since Arel is a part of Active Record now, I think that we can regard an
`ActiveModel::Attribute` object as boundable without wrapping it by a
`BindParam` to avoid extra `BindParam` allocation.