[stubgen] Always generate ellipsis for class attr with initial value

[michaelm-openai]

Fixes #16968

Currently semantic analysis is needed to detect whether a class is affected by dataclass transformation, which is the only case where where = ... is emitted for class attributes with default values besides named tuples.

This means that even if one adds explicit types everywhere they can in a file, they cannot rely on purely parse-based stub generation, because there is no way to explicitly specify a class attribute as needing an initializer in type stubs.

One approach could be to add a way to explicitly make that specification, but a much simpler approach is to always emit an ellipsis initializer if a class attribute is defined with an initial value. During typecheck, the ellipsis initializer would just have no effect if on a class not affected by dataclass transformation.

This idea is validated by Pyright, which already has this behavior using it's stub generation feature, i.e. pyright --createstub.