match_as_ref: improve diagnostics

[ada4a]

• Make the diagnostic message actually desribe the problem
• Make the suggestion verbose, because we lint multiline exprs

changelog: [match_as_ref]: improve diagnostics

[rustbot]

r? @samueltardieu

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[samueltardieu]

Note that this doesn't fix the bug in:

    let _: Option<&u32> = match Some(0) {
        //~^ match_as_ref
        None => None,
        Some(ref mut v) => Some(v),
    };

which will propose an incorrect fix.

View changes since this review

[ada4a]

@rustbot ready