clippy::comparison_chain false positive on PartialOrd

[timbodeit]

The comparison_chain lint is defined as:

Checks comparison chains written with if that can be rewritten with match and cmp.

However it also triggers on comparison chains written with if who's compared type does not implement Ord and doesn't offer cmp.

One such example would be f64. This type implements PartialOrd but not Ord. It provides < and > operators without providing cmp.
While in theory the same concept of using match could also be applied to the result of partial_cmp, I personally find matching over Optional<Ordering> much less appealing than matching over Ordering.

Should we apply the same lint to all PartialOrd cases and change the documentation to make this the official desired behavior? Or should we consider this a bug and change the implementation, so that the lint only fires if cmp is available on the relevant type (as specified in the documentation)?

cc @james9909

---

Example implementation where clippy::comparison_chain fires, eventough cmp is not available:

fn f(x: f64, y: f64) {
    if x > y {
        a()
    } else if x < y {
        b()
    } else {
        c()
    }
}

Reproduced on:
clippy 0.0.212 (3aea860 2019-09-03)
clippy 0.0.212 (4e7e71b 2019-10-11)

[flip1995]

We should only lint for types, that implement Ord. It would be possible to implement the same lint for PartialOrd, but put it in the pedantic group.