Use ConstArg for const param defaults

Now everything that actually affects the type system (i.e., excluding
const blocks, enum variant discriminants, etc.) *should* be using
`ConstArg`.

compiler/rustc_hir_pretty/src/lib.rs

@@ -2161,7 +2161,7 @@ impl<'a> State<'a> {
                 if let Some(default) = default {
                     self.space();
                     self.word_space("=");
-                    self.print_anon_const(default);
+                    self.print_const_arg(default);
                 }
             }
         }