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

[Balázs Benics via llvm-commits]

steakhal wrote:

Could you share us the benchmark result? I think it's worth to be checked in in the commit message for example.
That should pin how it looked when we added this.

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