Turns out that `doc_markdown` uses a non-cheap rustdoc function to
convert from markdown ranges into source spans. And it was using it a
lot (about once every 17 lines of documentation on `tokio`, which ends
up being about 2000 times).
This ended up being about 18% of the total Clippy runtime as discovered
by lintcheck --perf in docs-heavy crates. This PR optimizes one of the
cases in which Clippy calls the function, and a future PR once
pulldown-cmark/pulldown-cmark#1034 is merged will be opened. This PR
lands the use of the function into the single-digit zone.
Note that not all crates were affected by this crate equally, those with
more docs are affected far more than those light ones.
changelog:[`clippy::doc_markdown`] has been optimized by 50%
a6e40fa9e9