[libc++] __desugars_to_v doesn't recognize `const` arguments, leading to missed optimizations

A simple example is
```c++
auto test(int* first, int* last, const int* first2) {
  return std::equal(first, last, first2, std::less<int>());
}
```

`less<int>` doesn't get recognized here simply because the second pointer argument is `const`. We should fix that.
 