[clang] [analyzer] Improve handling of unsigned values in ArrayBoundCheckerV2 (PR #81034)

[via cfe-commits]

NagyDonat wrote:

> [...] we should strive to handle comparison evaluation more uniformly and preferably behind the API barrier of the constraint manager.

Actually this commit is a step towards the opposite direction -- it adds some tricky workaround logic to one particular checker (and not the constraint manager). Eventually it would be good to move this bounds checking into the constraint manager (or a related, but separate library file) -- but it's very hard to be clear, elegant and general in this area.

The main issue is that:
- due to the automatic conversion there is is a subtle difference between the C/C++ `operator<` and the mathematical _"less than"_,
- so it is not entirely correct to use `evalBinOpNN` to represent and check the preconditions of memory access,
- but we _want_ to use the same  symbolic expression to represent the comparisons expressed in the code (e.g. in an `if (idx < arraySize) {...}` branch) and the logical preconditions (that a given value must be in bounds)
- ...because otherwise a constraint coming from the conditional expression couldn't be used to prove the precondition.

This is why I _need_ to use this hacky "early return for the trivial cases, then use `evalBinOpNN` in the common case" strategy.

https://github.com/llvm/llvm-project/pull/81034