Verify that provision/omission of return types matches (#717)

This is finishing #678

executable_semantics/testdata/global_variable2.carbon

@@ -7,7 +7,7 @@
 
 var flag: i32 = 1;
 
-fn flipFlag() -> () {
+fn flipFlag() {
   flag = 0;
 }
 

executable_semantics/testdata/return_auto.carbon

@@ -0,0 +1,11 @@
+// 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 F() -> auto {
+  return 0;
+}
+
+fn main() -> i32 {
+  return F();
+}

executable_semantics/testdata/return_auto.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 5: syntax error, unexpected AUTO
+EXIT CODE: 255

executable_semantics/testdata/return_empty_explicit_fail.carbon

@@ -0,0 +1,12 @@
+// 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 F() {
+  return ();
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return_empty_explicit_fail.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: return (); should not provide a return value, to match the function's signature.
+EXIT CODE: 255

executable_semantics/testdata/return_empty_implicit_fail1.carbon

@@ -0,0 +1,11 @@
+// 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 F() -> () {
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return_empty_implicit_fail1.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+EXIT CODE: 255

executable_semantics/testdata/return_empty_implicit_fail2.carbon

@@ -0,0 +1,12 @@
+// 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 F() -> () {
+  return;
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return_empty_implicit_fail2.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: return; should provide a return value, to match the function's signature.
+EXIT CODE: 255