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

[David Blaikie via llvm-commits]

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

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.

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