Add semantic::Node

Rename Program(Builder)::Nodes to ASTNodes, and add SemNodes.

Bug: tint:390
Change-Id: If501ea5f5d1cbb4bc2673b303aa8ebce7195e2c3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/39005
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/semantic/node.h

@@ -0,0 +1,33 @@
+// Copyright 2021 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SRC_SEMANTIC_NODE_H_
+#define SRC_SEMANTIC_NODE_H_
+
+#include "src/castable.h"
+
+namespace tint {
+namespace semantic {
+
+/// Node is the base class for all semantic nodes
+class Node : public Castable<Node> {
+ public:
+  /// Destructor
+  ~Node() override;
+};
+
+}  // namespace semantic
+}  // namespace tint
+
+#endif  // SRC_SEMANTIC_NODE_H_