Make some Ordering methods const

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

Possible because of rust-lang#49146 (Allow `if` and `match` in constants).

library/core/src/cmp.rs

@@ -357,7 +357,7 @@ impl Ordering {
     #[inline]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn reverse(self) -> Ordering {
+    pub const fn reverse(self) -> Ordering {
         match self {
             Less => Greater,
             Equal => Equal,
@@ -395,7 +395,7 @@ impl Ordering {
     #[inline]
     #[must_use]
     #[stable(feature = "ordering_chaining", since = "1.17.0")]
-    pub fn then(self, other: Ordering) -> Ordering {
+    pub const fn then(self, other: Ordering) -> Ordering {
         match self {
             Equal => other,
             _ => self,