Warn when a never-return function is not written with -> !

[kmcallister]

! unifies with any type, so these are both valid:

fn foo() {
    loop { }
}

fn bar() -> uint {
    loop { }
}

Both would be clearer if written as -> !. (But skip this check when implementing trait methods, because their types are externally imposed.)

[ghost]

I'm gonna go ahead and close this in favour of #12836.