[llvm] [ADT] Simplify SmallPtrSetIterator (NFC) (PR #219087)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

 The description should say why the type-erasure argument from #160814 no longer applies. 

https://github.com/llvm/llvm-project/pull/219087