Add test for StructuralEq for std::cmp::Ordering.

Added test in library/core/tests/cmp.rs that ensures that `const`s of type `Ordering`s can be used in patterns.

Author: zachs18

library/core/tests/cmp.rs

@@ -133,6 +133,19 @@ fn ordering_const() {
     assert_eq!(THEN, Greater);
 }
 
+#[test]
+fn ordering_structural_eq() {
+    // test that consts of type `Ordering` are usable in patterns
+
+    const ORDERING: Ordering = Greater;
+
+    const REVERSE: Ordering = ORDERING.reverse();
+    match Ordering::Less {
+        REVERSE => {}
+        _ => unreachable!(),
+    };
+}
+
 #[test]
 fn cmp_default() {
     // Test default methods in PartialOrd and PartialEq