Add API to chain comment placement operations (#6319)
## Summary
This PR adds an API for chaining comment placement methods based on the
[`then_with`](https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then_with)
from `Ordering` in the standard library.
For example, you can now do:
```rust
try_some_case(comment).then_with(|comment| try_some_other_case_if_still_default(comment))
```
This lets us avoid this kind of pattern, which I've seen in
`placement.rs` and used myself before:
```rust
let comment = match handle_own_line_comment_between_branches(comment, preceding, locator) {
CommentPlacement::Default(comment) => comment,
placement => return placement,
};
```
