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

[via cfe-commits]

Sirraide wrote:

> So if there is some way to reduce the number of virtual methods, that would be good, but otherwise this just seems like the price of doing business...

I’ve already tried to remove as much functionality as possible, because some of the things that the RAV can do is only used by like, 4 visitors in the entire codebase (e.g. overriding `WalkUpFromX`), but more details on that on the pr for the DRAV impl.

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