Auto merge of #65939 - anp:incremental-rustfmt-rollout, r=Mark-Simulacrum

Auto merge of #65939 - anp:incremental-rustfmt-rollout, r=Mark-Simulacrum

Enable incremental rustfmt adoption

Enables an incremental rollout of rustfmt usage within the compiler via a granular ignore configuration and automated enforcement. The decision to format the repository was approved in https://github.com/rust-lang/compiler-team/issues/80#issuecomment-491324079.

This PR includes:

* an `[ignore]` section to `rustfmt.toml` including most of the repository
* `./x.py` downloads rustfmt from a specific nightly (we do not pin to beta or stable as we need unstable features)
* an `./x.py fmt [--check]` command which runs cargo-fmt
* `./x.py fmt --check` runs during the same test step as `src/tools/tidy`, on master, but not on beta or stable as we don't want to depend on nightly rustfmt there.
* a commit to format `src/librustc_fs_util` as an initial target and to ensure enforcement is working from the start