[clang] [clang] Fix the post-filtering heuristic for GSLPointer. (PR #114044)

[Haojian Wu via cfe-commits]

hokein wrote:

> Just discover a new false positive:
> 
> ```
> namespace std {
> template <typename T>
> class [[gsl::Pointer]] Iterator2 {
>  public:
>   using reference = T&;
>   Iterator2() {}
>   reference operator*() const;
> };
> }
> 
> template <typename T>
> class AnySpan {
>  public:
>   AnySpan() {}
>   std::Iterator2<T> begin() const [[clang::lifetimebound]];
> };
> 
> AnySpan<int> MakeAnySpan();
> void s() {
>   const int& t2 = *AnySpan<int>().begin();  // false positive, warning.
> }
> ```

Fix it in the latest commit https://github.com/llvm/llvm-project/pull/114044/commits/85e08706d4b9cfd6d419e486cdf1034c1dc1a45d, it would be nice to take another look at the patch, @Xazax-hun, @usx95.

https://github.com/llvm/llvm-project/pull/114044