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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

I think these are a net positive change; @nikic are you comfortable with the performance numbers? I feel like the changes come awfully close to falling out in the noise aside from the positive improvements with building Clang, but you look at compile time performance numbers more than I do, so you may have a better sense for whether the changes are acceptable.

Signing off with a LGTM, but please wait to land until @nikic has weighed in.

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