Do not report errors on skipped items or statements

[topecongiro]

Closes #1298.
Closes #2006.
Closes #2037.
cc #1978.

Note that this PR still warns on nested expressions with [rustfmt_skip]:

fn foo() {
    // Does not report an error.
    #[rustfmt_skip]
    let a = foo(
        vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27],
    );

    // Reports an error.
    let a = bar(
        #[rustfmt_skip] vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27],
    );
}

Handling nested expressions is hard because we do not use visitor for them. Assuming that most of the usage of #[rustfmt_skip] is on items or statements, I will send this PR anyway.

[nrc]

Awesome! Thank you!