[clang] [Clang][AST] Fix extending an unsigned to signed in `ExprConstant.cpp` (PR #180563)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I'd like to add a test for this:
> 
> ```c++
> int f(int __attribute__((vector_size(8))) v) {
>   return v[~0UL];
> }
> ```
> 
> Where should I put it?

I think `clang/test/SemaCXX/vector.cpp` or `clang/test/Sema/exprs.c` would be reasonable places for it. You should also add a release note to `clang/docs/ReleaseNotes.rst` as well.

https://github.com/llvm/llvm-project/pull/180563