[libc++] std::shared_ptr::reset() should catch being reset to its own pointer

https://compiler-explorer.com/z/s58er4o4z

```c++
#include <memory>

int main() {
    auto p = std::make_shared<int>(42);
    p.reset(p.get()); // Incorrect use of reset
}
```

libstdc++ catches that with

```
Assertion '__p == nullptr || __p != _M_ptr' failed.
```

I think it would be really easy to add this check in libc++ too.