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

[Arseniy Zaostrovnykh via llvm-commits]

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necto wrote:

Would it be useful to have an assertion checking that once the node equality is established, their paths are equal a well? This could help find some nasty bugs comparing iterators from different trees. It would be more expensive in debug builds, though, so I'll let you judge if it is worth it.

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