Make sure to run all clippy lints at warn level

[blyxyas]

Previously running all the Clippy bench suite with the general command

cargo run --bin collector profile_local perf-record ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --profiles Clippy --scenarios Full --exact-match ctfe-stress-5

This was because we were running deny-by-default lints and the collect is expecting everything to compile correctly.
Now rustc-perf sets every Clippy lint to warn to avoid that problem.

While it could be tempting to just open PRs to the respective failing crates to fix the issues, it's still healthy to run every single lint, even allow-by-defaults.

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Note that if you run rustc-perf with a non-master Clippy, it will still fail because there are some unfiltered CTFE checks. These will be removed in the next release so they're not really a problem.

cc @Kobzol

[Kobzol]

Thanks! Please reformat and I'll approve it.

[Kobzol]

Thanks! :)