Add unstable const_ordering feature, and some tests.

src/test/ui/consts/const-ordering.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(const_ordering)]
+
+use std::cmp::Ordering;
+
+// the following methods of core::cmp::Ordering are const:
+//  - reverse
+//  - then
+
+fn main() {
+    const REVERSE : Ordering = Ordering::Greater.reverse();
+    assert_eq!(REVERSE, Ordering::Less);
+
+    const THEN : Ordering = Ordering::Equal.then(REVERSE);
+    assert_eq!(THEN, Ordering::Less);
+}