Replace RTTI generation script with macros. (#2703)

This keeps all of our C++ code written in C++ source files, and avoids the increasing size of the RTTI generation script we'd started to see in #2699.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

explorer/ast/ast_rtti.cpp

@@ -0,0 +1,20 @@
+// 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 "explorer/ast/ast_rtti.h"
+
+namespace Carbon {
+
+// Define kind names for the base enumeration type.
+CARBON_DEFINE_ENUM_CLASS_NAMES(AstRttiNodeKind) = {
+    CARBON_AST_FOR_EACH_FINAL_CLASS(CARBON_ENUM_CLASS_NAME_STRING)};
+
+// For other kind enumerations, reuse the same table.
+#define DEFINE_NAME_FUNCTION(C)                                        \
+  CARBON_ENUM_NAME_FUNCTION(C##Kind) {                                 \
+    return AstRttiNodeKind(static_cast<const C##Kind&>(*this)).name(); \
+  }
+CARBON_AST_FOR_EACH_ABSTRACT_CLASS(DEFINE_NAME_FUNCTION)
+
+}  // namespace Carbon