[llvm] [ADT] Allow std::next to work on BitVector's set_bits_iterator (PR #80830)

[David Blaikie via llvm-commits]

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

It's OK (I have mixed feelings about the more integration-y test of using `std::next` rather than only testing the interface of `BitVector.h` to check the type traits directly, or whatever other features we should have - rather than whatever features `std::next` (on some implementation) happens to use - but it'll be OK as-is)

https://github.com/llvm/llvm-project/pull/80830