[clang] [Clang] [NFC] Introduce `DynamicRecursiveASTVisitor` (PR #105195)

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Sirraide wrote:

> I wonder if we can reduce binary size by simply marking RAV as `visibility("hidden")`

Hmm, I haven’t thought about that, but I’m not convinced it would help: the main issue w/ the RAV is that to traverse *every possible AST* that you might end up handing it, you need an instantiation of *the entire visitor*; I don’t think there is a good way to avoid that, so I don’t really see how that would help, but I’m also not really too familiar w/ how `visibility("hidden")` works (iirc it’s makes it so the symbol is not exported, but I think a lot of the visitors are already declared in an anonymous namespace or in a function).

https://github.com/llvm/llvm-project/pull/105195