Enforce in bootstrap that clippy must have stage at least 1

[Kobzol]

This mostly piggybacks on the previous x check rework.

The new "rules" follow the new staging logic. So x clippy <foo> lints foo using stage0 Clippy. x clippy --stage 2 <foo> lints foo using stage1 Clippy (which is built from in-tree sources).

I had to fix some latent issues with prepare_compiler_for_check along the way.

Checking rustc_private tools should now check less compiler crates (or rather not check compiler examples/tests/etc.), potentially speeding it up slightly.

I also had to switch x clippy ci to stage 2, so that it actually lints stuff using in-tree Clippy.

r? @jieyouxu

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[bors]

☔ The latest upstream changes (presumably #145011) made this pull request unmergeable. Please resolve the merge conflicts.

[jieyouxu]

Thanks, some questions

[jieyouxu]

@rustbot author

[Kobzol]

Rebased.

[jieyouxu]

Question: wait, this involves building stage 2 rustc, right...?

[jieyouxu]

^ this

[Kobzol]

Well, not building, but linting stage2 rustc. That should be the same as before, because we linted rustc (stage2) using in-tree Clippy (stage1).

[Kobzol]

Hmm, the log shows [build], I see xD I'll check.

[Kobzol]

I realized that x clippy ci is special enough to warrant a custom/manual selection of the invoked steps, effectively bypassing normal staging. I tried to optimize it to build as few things as possible.