Use a lambda for tag's visit (#645)

Also shifts declaration order a little for [style](https://google.github.io/styleguide/cppguide.html#Declaration_Order); moving line_num down (data members after methods) and tag() down (factory functions before instance members).

executable_semantics/interpreter/action.h

@@ -43,8 +43,6 @@ struct ValAction {
 };
 
 struct Action {
-  auto tag() const -> ActionKind;
-
   static auto MakeLValAction(const Expression* e) -> Action*;
   static auto MakeExpressionAction(const Expression* e) -> Action*;
   static auto MakeStatementAction(const Statement* s) -> Action*;
@@ -59,6 +57,10 @@ struct Action {
 
   void Print(std::ostream& out);
 
+  inline auto tag() const -> ActionKind {
+    return std::visit([](const auto& t) { return t.Kind; }, value);
+  }
+
   // The position or state of the action. Starts at 0 and goes up to the number
   // of subexpressions.
   //