Try using coverage tracking for Clippy

[phansch]

It would be nice if we were able to gather test coverage information for Clippy.

We have an old script written by @mcarton that could be used to run kcov on Clippy. It seems like this script doesn't work currently for various reasons.

While making coverage tracking work again it might be worth to investigate tarpaulin, as well.

@felix91gr mentioned on Discord that they would like to work on coverage tracking once #3963 is done.

[mati865]

There is no accurate coverage available for Rust right now. rust-lang/rust#34701 is promising but probably years from being done.

[ThibsG]

Recently, improvements have been done with the use of llvm-cov (see rust-lang/rust#34701).

Zulip thread to see rendering work done by @flip1995.

To improve this rendering, one leftover question is how to exclude dependencies from the results (e.g. exclude results from .cargo and .rustup paths): maybe building deps without -Zinstrument-coverage?