Python: Prevent bad join in `isinstanceEvaluatesTo`
Python: Prevent bad join in `isinstanceEvaluatesTo`

In some cases, we were joining the result of `val.getClass()` against
the first argument of `Types::improperSubclass` before filtering out the
vast majority of tuples by the call to `isinstance_call`.

To fix this, we let `isinstance_call` take care of figuring out the
class of the value being tested. As a bonus, this cleans up the only
other place where `isinstance_call` is used, where we _also_ want to
know the class of the value being tested in the `isinstance` call.

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