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

[via cfe-commits]

martinboehme wrote:

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.

https://github.com/llvm/llvm-project/pull/89596