[llvm] [SmallPtrSet] Don't leave tombstones in small mode (PR #96762)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Nice improvement! Left some comments regarding the documentation; we should also update the programmers manual: https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallptrset-h

https://github.com/llvm/llvm-project/pull/96762