Auto merge of #56595 - ljedrz:x_py_clippy_fix, r=oli-obk

Add clippy and fix commands to x.py

Since they are kind of similar in nature, I have used the same approach as for `cargo check`. At least some of the boilerplate could probably be shared, but I'd prefer to gather some feedback before I decide to merge them more aggressively.

This works reasonably well for `clippy`; with `-A clippy::all` and some extra `#![feature(rustc_private)]`s almost the whole codebase can be processed. There are some concerns, though:
- unlike `check`, in order to be able to traverse all the crates, some of them need to be marked with the `#![feature(rustc_private)]` attribute
- `-W clippy::all` breaks on any error. Is there a way to produce errors but not have them break the progress?
- I'm not sure how to redirect the errors in a way that would show colors; for now I was able to de-jsonize and print them (something not needed for `check`)

`cargo fix` is much more stubborn; it refuses to acknowledge crates like `core` and `std`, so it doesn't progress much at all.

Since this is a bit more tricky than I have envisioned, I need some guidance:
- is this the right approach or am I doing something very wrong ^^?
- why are the extra `rustc_private` features necessary? I was hoping for the same treatment as `check`
- are changes in `clippy` and `cargo fix` needed e.g. in order to produce errors in the same manner as `check` or did I miss something?
- do we need this level of file granularity (e.g. for futureproofing) or can `check`, `clippy` and `fix` files be condensed?

Hopes-to-fix: https://github.com/rust-lang/rust/issues/53896

Cc @alexcrichton, @zackmdavis

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