Rollup merge of #144027 - RalfJung:clippy, r=Mark-Simulacrum

clippy: make tests work in stage 1

This finally fixes rust-lang/rust#78717 :)

Similar to what Miri already does, the clippy test step needs to carefully consider  which compiler is used to build clippy and which compiler is linked into clippy (and thus must be used to build the test dependencies). On top of that we have some extra complications that Miri avoided by using `cargo-miri` for building its test dependencies: we need cargo to use the right rustc and the right sysroot, but rust-lang/cargo#4423 makes this quite hard to do. See the long comment in `src/tools/clippy/tests/compile-test.rs` for details.

Some clippy tests tried to import rustc crates; that fundamentally requires a full bootstrap loop so it cannot work in stage 1. I had to kind of guess what those tests were doing so I don't know if my changes there make any sense.

Cc ```@flip1995``` ```@Kobzol```

Author: jieyouxu