Auto merge of #50930 - toidiu:ak-ordOutlivesPredicate, r=nikomatsakis

implement Ord for OutlivesPredicate and other types

It became necessary while implementing https://github.com/rust-lang/rust/pull/50070 to have `Ord` implemented for `OutlivesPredicate`.

This PR implements `Ord` for `OutlivesPredicate` as well as other types needed for the implementation.

src/libsyntax/ast.rs

@@ -667,7 +667,7 @@ pub enum PatKind {
     Mac(Mac),
 }
 
-#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
 pub enum Mutability {
     Mutable,
     Immutable,