[clang] [Clang] [NFC] Introduce `DynamicRecursiveASTVisitor` (PR #110040)

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+//===--- DynamicRecursiveASTVisitor.h - Virtual AST Visitor -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file defines the DynamicRecursiveASTVisitor interface, which acts
+//  identically to RecursiveASTVisitor, except that it uses virtual dispatch
+//  instead of CRTP, which greatly improves compile times and  binary size.
+//
+//  However, it also comes with limitations in that some of the more seldom
+//  utilised features of RecursiveASTVisitor are not supported.
+//
+//  Prefer to use this over RecursiveASTVisitor whenever possible.
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_CLANG_AST_DYNAMIC_RECURSIVE_AST_VISITOR_H
+#define LLVM_CLANG_AST_DYNAMIC_RECURSIVE_AST_VISITOR_H
+
+#include "clang/AST/Attr.h"
+#include "clang/AST/ExprConcepts.h"
+#include "clang/AST/TypeLoc.h"
+
+namespace clang {
+class ASTContext;
+
+/// Recursive AST visitor that supports extension via dynamic dispatch.
+///
+/// This only supports some of the more common visitation operations; in
+/// particular, it does not support overriding WalkUpFromX or post-order
+/// traversal.
+///
+/// Instead of functions (e.g. shouldVisitImplicitCode()), this class
+/// uses member variables (e.g. ShouldVisitImplicitCode) to control
+/// visitation behaviour.
+///
+/// RAV features that are NOT supported:
+///
+///   - Visiting attributes,
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Sirraide wrote:

> What is the behavior when visiting an attribute?

Ah, no, ‘not supported’ is supposed to mean that *overriding* them is not supported—I should probably make that a bit clearer in the comment. You can still use a DRAV to visit attributes just fine—you just can’t override what happens when one is visited (i.e. the default RAV implementation is called).

https://github.com/llvm/llvm-project/pull/110040