[clang] [analyzer] EvalBinOpLL should return Unknown less often (PR #114222)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> I think we don't have any mechanisms like these. This was just a theoretical comment if I understood it right.

Actually, both. Yes, we don't have many of these in the engine as of today, but we do model some overflows in some checkers, like the bitwise shifts checker. It would be nice if we could write that logic once, and reuse it in constant folding as well, but this is really theoretical at this point. 

https://github.com/llvm/llvm-project/pull/114222