They can just be written as `use rustc_foo::bar`, which is far more
standard. (I didn't even know that a `crate::` prefix was valid.)

…Nilstrieb

Remove braces when fixing a nested use tree into a single item

[Back in 2019](rust-lang#56645) I added rustfix support for the `unused_imports` lint, to automatically remove them when running `cargo fix`. For the most part this worked great, but when removing all but one childs of a nested use tree it turned `use foo::{Unused, Used}` into `use foo::{Used}`. This is slightly annoying, because it then requires you to run `rustfmt` to get `use foo::Used`.

This PR automatically removes braces and the surrouding whitespace when all but one child of a nested use tree are unused. To get it done I had to add the span of the nested use tree to the AST, and refactor a bit the code I wrote back then.

A thing I noticed is, there doesn't seem to be any `//@ run-rustfix` test for fixing the `unused_imports` lint. I created a test in `tests/suggestions` (is that the right directory?) that for now tests just what I added in the PR. I can followup in a separate PR to add more tests for fixing `unused_lints`.

This PR is best reviewed commit-by-commit.

…r=dtolnay

Generic `NonZero` post-stabilization changes.

Tracking issue: rust-lang#120257

r? `@dtolnay`

crashes: add lastest batch of crash tests

Make sure we don't deny macro vars w keyword names

`$async:ident`, etc are all valid.

Fixes rust-lang#124862

…, r=compiler-errors

Simplify `use crate::rustc_foo::bar` occurrences.

They can just be written as `use rustc_foo::bar`, which is far more standard. (I didn't even know that a `crate::` prefix was valid.)

r? `@eholk`

Update cc crate to v1.0.97