Add clippy::result_as_ref_deref lint

[aleksanderkrauze]

changelog: added new lint result_as_ref_deref

closes #13342

I've added new lint result_as_ref_deref, which mirrors already existing lint option_as_ref_deref and reused its existing check function.

Open question is should this be another lint, or should two of them be merged together? I've asked about it in the original issue (proposing manual_as_ref_deref name), and @Jarcho suggested the same thing (and proposed manual_fallible_as_deref name).

I'm not sure what is clippy's policy regarding renaming lint names, so I opted into just adding new lint and waiting for the review.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Jarcho (or someone else) some time within the next two weeks.

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[aleksanderkrauze]

Uh. I think I messed something up. I've pulled master branch to be up to date, rebased my working branch on top of it, and now tests don't pass and a whole bunch of code needs to be reformatted. However when I look at the diff between my branch and upstream/master I only see my changes. Help would be appreciated. 😃

EDIT. After some more rebasing and tweaking all seams to work now. I don't know what I did previously wrong, but now CI passes.

[samueltardieu]

Using get_type_diagnostic_name() would let you simplify this part:

    let target_ty = cx.typeck_results().expr_ty(as_ref_recv);
    let target = match get_type_diagnostic_name(cx, target_ty) {
        Some(sym::Option) if msrv.meets(msrvs::OPTION_AS_DEREF) => Target::Option,
        Some(sym::Result) if msrv.meets(msrvs::RESULT_AS_DEREF) =>Target::Result,
        _ => return,
    };