option to enforce ordering for attributes and doc comments

There are two orderings possible with items that have both attributes and doc comments

/// doc comment
#[attribute]
fn foo() {}

#[attribute]
/// doc comment
fn foo() {}

A random twitter poll that I cannot find anymore made it clear that option 1 is significantly more popular than option 2 and some people (myself included) would prefer if we just settled on one of these variants and consistently enforced that style.

The proposed feature is to add a check for items that have both a doc comment and attributes and to format them all consistently. There should be an option to select either style. I believe that the former should be the default setting.

Start by locating rustfmt's handling of items with both attributes and doc comments, along with its existing configuration options and tests. Implement the proposed check with selectable ordering, using the preferred doc-comment-first style as the default; done means both variants are formatted consistently according to that option.