Support complex type patterns in bindings. (#759)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/tuple5.golden

@@ -1,4 +1,2 @@
-COMPILATION ERROR: 8: type error in pattern variable
-expected: (x = i32, y = i32)
-actual: (y = i32, x = i32)
+PROGRAM ERROR: 8: Tuple field name 'y' does not match pattern field name 'x'
 EXIT CODE: 255

executable_semantics/testdata/type_match.carbon

@@ -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() -> i32 {
+  var t: (auto, (i32, i32)) = ((1,2),(3,4));
+  return t[0][0] + t[1][1] - 5;
+}

executable_semantics/testdata/type_match.golden

@@ -0,0 +1 @@
+result: 0