[clang] [Clang] [NFC] Introduce `ConstDynamicRecursiveASTVisitor` (reland) (PR #124821)

[via cfe-commits]

Sirraide wrote:

> Well, at worst
> 
> ```c++
> using Base = RecursiveASTVisitor<Impl<Const>>; // and you can put that at class level
> return Base::Function(...);
> ```

It’d have to be `Impl<Const>::Base::Function(...)` because RAV is only a base class of `Impl` not of DRAV; that would still have a dependent NNS, wouldn’t it?

https://github.com/llvm/llvm-project/pull/124821