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/ast/declaration.cpp

@@ -28,4 +28,13 @@ void ChoiceDeclaration::Print() const {
   std::cout << "}" << std::endl;
 }
 
+// Print a global variable declaration to standard out.
+void VariableDeclaration::Print() const {
+  std::cout << "var ";
+  PrintExp(type);
+  std::cout << " : " << name << " = ";
+  PrintExp(initializer);
+  std::cout << std::endl;
+}
+
 }  // namespace Carbon