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

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

> unsigned int foo(unsigned char x)
> {
>     return ~(1<<x); // warning: implicit conversion loses integer precision: 'int' to 'unsigned int'
> }
> ```

Could you help me understand why clang is emitting the warning?

Isn't it semantically equivalent to the following code?

```
unsigned int foo(unsigned char x)
{
     int i = ~(1<<x); // no -Wimplicit-int-conversion warning.
     return I; // no -Wimplicit-int-conversion warning.
}
```

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