Rollup merge of #127950 - nnethercote:rustfmt-skip-on-use-decls, r=cuviper

Use `#[rustfmt::skip]` on some `use` groups to prevent reordering.

`use` declarations will be reformatted in #125443. Very rarely, there is a desire to force a group of `use` declarations together in a way that auto-formatting will break up. E.g. when you want a single comment to apply to a group. #126776 dealt with all of these in the codebase, ensuring that no comments intended for multiple `use` declarations would end up in the wrong place. But some people were unhappy with it.

This commit uses `#[rustfmt::skip]` to create these custom `use` groups in an idiomatic way for a few of the cases changed in #126776. This works because rustfmt treats any `use` item annotated with `#[rustfmt::skip]` as a barrier and won't reorder other `use` items around it.

r? `@cuviper`

Author: matthiaskrgr