Tuple restrictions (#486)

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

* after_named_member => seen_named_member

executable_semantics/testdata/tuple5.6c

@@ -0,0 +1,10 @@
+// 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
+
+// Test the that field order matters for tuples.
+
+fn main() -> Int {
+  var (.x = Int, .y = Int): t = (.y = 2, .x = 3);
+  return 0;
+}