Rollup merge of #76198 - CDirkx:const-ordering, r=dtolnay

Rollup merge of #76198 - CDirkx:const-ordering, r=dtolnay

Make some Ordering methods const

Resubmission of [PR#75463](https://github.com/rust-lang/rust/pull/75463) as per [PR#76172](https://github.com/rust-lang/rust/pull/76172).

Constify the following methods of `core::cmp::Ordering`:
 - `reverse`
 - `then`

Insta-stabilizes these methods as const under the `const_ordering` feature, as their implementation is a trivial match and the recent stabilization of #49146 (Allow `if` and `match` in constants).
Note: the `const_ordering` feature has never actually been used as these methods have not been `#[rustc_const_unstable]`.

Tracking issue:  #76113