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/ast/expression.cpp

@@ -8,21 +8,6 @@
 
 namespace Carbon {
 
-namespace {
-
-struct TagVisitor {
-  template <typename Alternative>
-  auto operator()(const Alternative&) -> ExpressionKind {
-    return Alternative::Kind;
-  }
-};
-
-}  // namespace
-
-auto Expression::tag() const -> ExpressionKind {
-  return std::visit(TagVisitor(), value);
-}
-
 auto Expression::GetIdentifierExpression() const
     -> const IdentifierExpression& {
   return std::get<IdentifierExpression>(value);