## Summary
This PR leverages the `Arguments` AST node introduced in #6259 in the
formatter, which ensures that we correctly handle trailing comments in
calls, like:
```python
f(
1,
# comment
)
pass
```
(Previously, this was treated as a leading comment on `pass`.)
This also allows us to unify the argument handling across calls and
class definitions.
## Test Plan
A bunch of new fixture tests, plus improved Black compatibility.
4c53bfe896