Rollup merge of #156466 - kpreid:checkattr, r=JonathanBrouwer

Refactor `CheckAttrVisitor` so rustfmt can format it.

I don’t have any need for this myself, but [I heard it was annoying people](https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/.60.23.5Bdiagnostic.3A.3Aon_unimplemented.5D.60.20on.20trait.20methods.3F/near/594113991) and I like fixing formatting and structure problems.

The key change is that the do-nothing cases are now individual match arms instead of an enormous or-pattern. In order to achieve this cleanly, I moved the code handling `Attribute::Parsed` into a separate function matching `AttributeKind`s rather than `Attribute`s.

This also fixes `RustcAllowIncoherentImpl` being non-alphabetical because it was spelled without a leading “|” (though that by itself could be achieved by adding the optional leading “|”).