[clang] [Clang] Force expressions with UO_Not to not be non-negative (PR #126846)

[Yutong Zhu via cfe-commits]

YutongZhuu wrote:

> > > But still I feel generate a warning for this case went too far.
> > 
> > 
> > Yeah, that's probably right. Maybe for `-` on a signed operand, we should just return the original range with the `NonNegative` flag cleared out, and shouldn't add the extra bit for the `-128 -> 128` edge case. That's not technically correct, but probably is more useful in practice.
> 
> Hm. That change will reintroduce a false positive warning for:
> 
> ```c++
> bool b(signed char c) {
>   return -c >= 128;
> }
> ```
> 
> ... that this patch fixed. But we don't produce a false positive for `0 - c >= 128`, so I still think what we ought to do here is to make `-c` behave the same way that `0 - c` does.

I don't think I can fix the problem without re-introducing the false positive in ``TryGetExprRange``.  By looking at the AST, the only difference is that 
```c++
unsigned char b = -a 
```
has a implCast to unsigned char right before the unary operator but the false positive case does not. By the time that the expression is passed into ``TryGetExprRange``,  the information about the implCast has already lost (And I cannot find any APIs that is able to get the parent expression or something like that). I think it would be better to re-introduce the false positive because I think having the implicit-int-conversion warning sounds more unacceptable. Or you have any other suggestions on differentiating between the two cases?

https://github.com/llvm/llvm-project/pull/126846