[libc++] ADL-proof <iterator>. `__convert_to_integral` is not a customization point.

The interesting change here is that we no longer consider `__convert_to_integral`
an ADL customization point for the user's types. I think the new behavior
is defensible. The old behavior had come from D7449, where Marshall explicitly
said "people can't define their own [`__convert_to_integral` overloads]."

Differential Revision: https://reviews.llvm.org/D92814

GitOrigin-RevId: c0428b3c0c1f3b78d39ceaf909908800fb7aabe3
diff --git a/include/iterator b/include/iterator
index 90b5f41..3e1fb61 100644
--- a/include/iterator
+++ b/include/iterator
@@ -663,9 +663,9 @@
 {
     _LIBCPP_ASSERT(__orig_n >= 0 || __is_cpp17_bidirectional_iterator<_InputIter>::value,
                    "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
-    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
-    __advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
+    _VSTD::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
 }
 
 template <class _InputIter>
@@ -692,7 +692,7 @@
 typename iterator_traits<_InputIter>::difference_type
 distance(_InputIter __first, _InputIter __last)
 {
-    return __distance(__first, __last, typename iterator_traits<_InputIter>::iterator_category());
+    return _VSTD::__distance(__first, __last, typename iterator_traits<_InputIter>::iterator_category());
 }
 
 template <class _InputIter>