Reapply "[clang][dataflow] Model conditional operator correctly." with fixes (#89596)

I reverted https://github.com/llvm/llvm-project/pull/89213 beause it was
causing buildbots to fail with assertion failures.

Embarrassingly, it turns out I had been running tests locally in
`Release` mode, i.e. with `assert()` compiled away.

This PR re-lands #89213 with fixes for the failing assertions.