Issue with repr() for differential components on frames after set_representation_cls

Found a bug in `BaseCoordinateFrame._data_repr()`. I haven't tracked down exactly what the issue is, but here's how to reproduce:

```python
>>> import astropy.coordinates as coord
>>> import astropy.units as u
>>> c = coord.ICRS(ra=150*u.deg, dec=-11*u.deg,
... pm_ra_cosdec=100*u.mas/u.yr,
... pm_dec=199*u.mas/u.yr)
>>> c

>>> c.set_representation_cls(s=coord.CartesianDifferential)
>>> c

```

After setting the differential class, the repr for the differential components reverts to the `UnitSphericalCosLatDifferential` component names. This has something to do with the
```python
if isinstance(dif_data, (r.UnitSphericalDifferential,
r.UnitSphericalCosLatDifferential,
r.RadialDifferential)):
```
in `_data_repr`.

cc @mhvk

Start at BaseCoordinateFrame._data_repr and run the reproduction using ICRS, set_representation_cls, and CartesianDifferential. Trace the differential-type handling around the isinstance check; done means the repr preserves the selected differential component names instead of reverting to UnitSphericalCosLatDifferential names.

Written by the indexing model from the issue text.