Lint for loops that will never run more than once

[nox]

Clippy should warn about loop { return; }.

[Havvy]

I'm going to take my hand at this.

[mcarton]

@Havvy Thanks! Feel free to ask if you have any question.

[Havvy]

Blocked on rust-lang/rust#37535

Well, theoretically anyways. I more want those methods there because they then have a guarantee that each node will have its NodeIndex with it in the iterator.

[Havvy]

Also, I'm using a CFG analysis for this, not AST. So T-AST is wrong. As is E-easy. E-medium perhaps.

[camsteffen]

Resolved by #1549.