Rationale:
- `atty` is widely used in the Rust ecosystem
- We already use it (in `rustc_errors` and other places)
- We shouldn't be rolling our own TTY detector when there's a
widely-used, well-tested package that we can use
The primary motivation is to get the changes from
https://github.com/tokio-rs/tracing/pull/990. Example output:
```
$ RUSTDOC_LOG=debug rustdoc +rustc2
warning: some trace filter directives would enable traces that are disabled statically
| `debug` would enable the DEBUG level for all targets
= note: the static max level is `info`
= help: to enable DEBUG logging, remove the `max_level_info` feature
```
- Remove useless test
This was testing for an ICE when passing `RUST_LOG=rustc_middle`. I
noticed it because it started giving the tracing warning (because tests
are not run with debug-logging enabled). Since this bug seems unlikely
to re-occur, I just removed it altogether.
