[libc++] Use __alloc_traits in <deque> whenever it is available for consistency (#126595)

When an allocator-aware container already defines a member type alias
`__alloc_traits` for `std::allocator_traits<allocator_type>`, we should
consistently use `__alloc_traits`. Mixing the usage of both
`__alloc_traits` and `std::allocator_traits` can lead to inconsistencies
and confusion.

Author: winner245

libcxx/include/deque

@@ -59,7 +59,7 @@ public:
 
     deque& operator=(const deque& c);
     deque& operator=(deque&& c)
-        noexcept((__alloc_traits::propagate_on_container_move_assignment::value &&
+        noexcept((allocator_traits<allocator_type>::propagate_on_container_move_assignment::value &&
                   is_nothrow_move_assignable<allocator_type>::value) ||
                  allocator_traits<allocator_type>::is_always_equal::value);
     deque& operator=(initializer_list<value_type> il);
@@ -677,7 +677,7 @@ public:
   _LIBCPP_HIDE_FROM_ABI deque& operator=(deque&& __c) noexcept(
       (__alloc_traits::propagate_on_container_move_assignment::value &&
        is_nothrow_move_assignable<allocator_type>::value) ||
-      allocator_traits<allocator_type>::is_always_equal::value);
+      __alloc_traits::is_always_equal::value);
 
   _LIBCPP_HIDE_FROM_ABI void assign(initializer_list<value_type> __il) { assign(__il.begin(), __il.end()); }
 #  endif // _LIBCPP_CXX03_LANG
@@ -1382,7 +1382,7 @@ template <class _Tp, class _Allocator>
 inline deque<_Tp, _Allocator>& deque<_Tp, _Allocator>::operator=(deque&& __c) noexcept(
     (__alloc_traits::propagate_on_container_move_assignment::value &&
      is_nothrow_move_assignable<allocator_type>::value) ||
-    allocator_traits<allocator_type>::is_always_equal::value) {
+    __alloc_traits::is_always_equal::value) {
   __move_assign(__c, integral_constant<bool, __alloc_traits::propagate_on_container_move_assignment::value>());
   return *this;
 }