feat(syntax): RTN in bounds

Limited syntactic support for experimental return type notations.
https://github.com/rust-lang/rust/issues/109417

crates/hir-def/src/path/lower.rs

@@ -218,6 +218,9 @@ pub(super) fn lower_generic_args(
                 let arg = ConstRefOrPath::from_expr_opt(arg.expr());
                 args.push(GenericArg::Const(arg))
             }
+            ast::GenericArg::ReturnTypeArg(_) => {
+                // TODO: return type notation is experimental, we don't do anything with it yet.
+            }
         }
     }