[llvm] [ADT] Add back ability to compare StringSet (PR #91374)

[Aleksandr Platonov via llvm-commits]

ArcsinX wrote:

> Looks OK to me - bit of a weird special case for StringMap, so an alternative might be for StringSet to define its own, always-true equality comparable, value type to use like it did before. But I wouldn't insist on that.

I thought about this, but it seems to me that it would be a bit of a step back. In such an approach we need to add a type similar to `llvm::NoneType`, which was previously removed as unnecessary.

https://github.com/llvm/llvm-project/pull/91374