Cheap `derive(PartialOrd)` for non-generic types

[nnethercote]

If a non-generic type derives both PartialOrd and Ord, the PartialOrd impl can just do this:

impl PartialOrd for T {
    fn_partial_cmp(&self, other: &T) {
        Some(self.cmp(other))
    }
}

(It won't work for generic types because the trait bounds on the generic parameters won't be right.)

This will likely be faster to compiler than a full implementation. It might also be a little easier for the backend to optimize.

Some futher discussion is on Zulip here.

[scottmcm]

Note that we do something similar for Copy+Clone -- see a demo https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=7ce592ad4d8e294b193837d78485e7c6 by clicking "Tools -> Expand Macros".

This will need the same kind of "can only do the special case if the type is non-generic" that we do for those.