Tuple restrictions (#486)

* implement tuple restrictions: (1) named fields after positional, (2) order matters

* after_named_member => seen_named_member

executable_semantics/testdata/tuple4.6c

@@ -0,0 +1,8 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> Int {
+  var auto: t = (.x = 2, 3);
+  return 0;
+}