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

[Tim Gymnich via llvm-commits]

https://github.com/tgymnich requested changes to this pull request.

for the curious: https://godbolt.org/z/c9sMMn665

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