Fix equality for tuples (#446)

* fix tuple equality, added test cases

* added a comment to an old function

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Geoff Romer <gromer@google.com>

* fix error in Geoffrey's edit

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/tuple_equality.6c

@@ -0,0 +1,13 @@
+// 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 (Int,Int): t1 = (5, 2);
+  var (Int,Int): t2 = (5, 2);
+  if (t1 == t2) {
+    return 0;
+  } else {
+    return 1;
+  }
+}

executable_semantics/testdata/tuple_equality.golden

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

executable_semantics/testdata/tuple_equality2.6c

@@ -0,0 +1,13 @@
+// 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 (Int,Int): t1 = (5, 2);
+  var (Int,Int): t2 = (5, 4);
+  if (t1 == t2) {
+    return 1;
+  } else {
+    return 0;
+  }
+}

executable_semantics/testdata/tuple_equality2.golden

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

executable_semantics/testdata/tuple_equality3.6c

@@ -0,0 +1,13 @@
+// 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 (Int,Int): t1 = (5, 2);
+  var (Int,Int): t2 = (5,);
+  if (t1 == t2) {
+    return 1;
+  } else {
+    return 0;
+  }
+}

executable_semantics/testdata/tuple_equality3.golden

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