[llvm] [ADT] Simplify SmallSetIterator with std::variant (NFC) (PR #157229)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

In addition to the concern about an extra include, I haven't seen `std::variant` used in llvm/mlir but heard that some implementations are not very peformant. I think we'd want to proceed with caution in case doesn't end up being just an NFC.

https://github.com/llvm/llvm-project/pull/157229