[llvm] [MC/DC] Introduce `class TestVector` with a pair of `BitVector` (PR #82174)

[Jessica Paquette via llvm-commits]

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

I think the code looks fine. I left a couple nits on comments. But otherwise, LGTM.

https://github.com/llvm/llvm-project/pull/82174