Global variables (#378)

* global variables

* implemented type checking of global variable, added test case

* added a comment

* improvements based on Dave's suggestions

* improvements based on Jon's suggestions

* added test cases about global variable ordering

executable_semantics/testdata/global_variable2.6c

@@ -0,0 +1,17 @@
+// 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 that mutations to a global variable in one function is visible
+// in another function.
+
+var Int: flag = 1;
+
+fn flipFlag() -> () {
+  flag = 0;
+}
+
+fn main() -> Int {
+  flipFlag();
+  return flag;
+}