[clang] [analyzer] Don't assume third iteration in loops (PR #119388)

[Balazs Benics via cfe-commits]

https://github.com/steakhal commented:

Thank you for the ping. I'm really busy lately with stabilizing Z3 refutation.

I really liked the extensive evaluation, which helped a lot forming my opinion here.
The diffs look all right. I only spotchecked some places the reports. I agree with your evaluation.

The content of the patch looks also correct. I only found a couple Grammarly spellings, but that's it.
I wonder how you found out that loop widening may conflict with this heuristic, were any tests broken that raised your awareness?

I have one concern with `didEagerlyAssumeBifurcateAt`, that it is bound and overwritten at the most recent eager split - which to my knowledge may not be the loop condition if we have something like this:
```c++
while (coin()) {
  if (coin()) { // #inner_coin
    ++num;
  }
}
```
Each time the `inner_coin` would spoil the `LastEagerlyAssumeBifurcationAt` trait between the loop iterations, right?

Storing `const Expr *` pointers is brittle for recursive functions too, where the same expression could appear in different LocationContexts. I wonder if we could make an example where it would confuse this heuristic.

I'm also not entirely convinced about nested loops - if this heuristic would kick in. This is basically a variant of the `coin` case I showed an example.

I don't have a counter example just yet, but I'm not sure I can spare the time to craft one.

Once again, big kudos for the extensive data. And I have no objections overall, I only have these minor questions before I'd approve this.

https://github.com/llvm/llvm-project/pull/119388