[llvm] [ADT] Rewrite ImmutableSet/Map in-order iterator without per-node state (PR #205552)

[Arseniy Zaostrovnykh via llvm-commits]

necto wrote:

> @necto could you take a look if this helps the analyzer performance on your benchmarks?

The performance boost sounds exciting, thanks! I will try to find some time to benchmark it on our downstream fork, but don't wait on it.

https://github.com/llvm/llvm-project/pull/205552