Define a base class for all AST nodes. (#947)

Also implement code-generation to manage the resulting boilerplate.

executable_semantics/ast/ast_node.cpp

@@ -0,0 +1,11 @@
+// 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
+
+#include "executable_semantics/ast/ast_node.h"
+
+namespace Carbon {
+
+AstNode::~AstNode() = default;
+
+}  // namespace Carbon