Using loc[] or at[] to add rows doesn't work with an IntervalIndex doesn't work
It seems that a data frame with an `IntervalIndex` won't let you use `loc[]` or `at[]` to add rows.

I would expect the following to work:
```
df = pd.DataFrame(index=pd.IntervalIndex(closed='left', dtype='interval[int64]',
                                         data=[pd.Interval(0, 1, closed='left')]),
                  data={'a': [42]})
df.at[pd.Interval(2, 3, closed='left'), 'a'] = 43
```

However, it throws a confusing error
```
AttributeError: 'pandas._libs.interval.IntervalTree' object has no attribute 'set_value'
```