Change how anonymous associated types are printed.

Give them their own symbol `anon_assoc`, as is done for all the other
anonymous `DefPathData` variants.

compiler/rustc_hir/src/definitions.rs

@@ -446,7 +446,7 @@ impl DefPathData {
             Ctor => DefPathDataName::Anon { namespace: sym::constructor },
             AnonConst => DefPathDataName::Anon { namespace: sym::constant },
             OpaqueTy => DefPathDataName::Anon { namespace: sym::opaque },
-            AnonAssocTy => DefPathDataName::Anon { namespace: sym::synthetic },
+            AnonAssocTy => DefPathDataName::Anon { namespace: sym::anon_assoc },
             SyntheticCoroutineBody => DefPathDataName::Anon { namespace: sym::synthetic },
         }
     }