Add support for return; (#678)

This creates a ReturnExpression. A separate change should enforce that an implicit return of `()` is only allowed in functions that have an implicit return type of `()`, and I think the structure taken here should ease that.

executable_semantics/testdata/return_empty_explicit.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() -> Int {
+  F();
+  return 0;
+}

executable_semantics/testdata/return_empty_explicit.golden

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

executable_semantics/testdata/return_empty_implicit1.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() -> Int {
+  F();
+  return 0;
+}

executable_semantics/testdata/return_empty_implicit1.golden

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

executable_semantics/testdata/return_empty_implicit2.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() -> Int {
+  F();
+  return 0;
+}

executable_semantics/testdata/return_empty_implicit2.golden

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