Assign more diagnostic codes

These macros may actually be best written as this:

```rust
macro_rules! span_foo(
    ($session:expr, $span:expr, $code:ident, $($message:tt)*) => ({
        __diagnostic_used!($code);
        $session.span_warn_with_code($span, format!($($message)*), stringify!($code))
    })
)
```

That way you'll handle things like `foo=bar` as well and you won't need duplicate cases for arguments and no arguments.