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_variable8.6c

@@ -0,0 +1,14 @@
+// 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 a global variable may not depend on a later global.
+// Error expected.
+
+var Int: x = y;
+
+var Int: y = 0;
+
+fn main() -> Int {
+  return x;
+}