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

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,268 @@
+//===--- 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.
----------------
AaronBallman wrote:

```suggestion
//  instead of CRTP, which greatly improves compile times and binary size.
```

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