[libc++] Consistently replace `std::` qualification with `_VSTD::` or nothing. NFCI.

I used a lot of `git grep` to find places where `std::` was being used
outside of comments and assert-messages. There were three outcomes:

- Qualified function calls, e.g. `std::move` becomes `_VSTD::move`.
    This is the most common case.

- Typenames that don't need qualification, e.g. `std::allocator` becomes `allocator`.
    Leaving these as `_VSTD::allocator` would also be fine, but I decided
    that removing the qualification is more consistent with existing practice.

- Names that specifically need un-versioned `std::` qualification,
    or that I wasn't sure about. For example, I didn't touch any code in
    <atomic>, <math.h>, <new>, or any ext/ or experimental/ headers;
    and I didn't touch any instances of `std::type_info`.

In some deduction guides, we were accidentally using `class Alloc = typename std::allocator<T>`,
despite `std::allocator<T>`'s type-ness not being template-dependent.
Because `std::allocator` is a qualified name, this did parse as we intended;
but what we meant was simply `class Alloc = allocator<T>`.

Differential Revision: https://reviews.llvm.org/D92250

GitOrigin-RevId: d586f92c9456a972ee475a021525c0522b89587b
diff --git a/include/numeric b/include/numeric
index ed06fcc..4f202bb 100644
--- a/include/numeric
+++ b/include/numeric
@@ -377,7 +377,7 @@
                                _OutputIterator __result, _BinaryOp __b)
 {
     if (__first != __last) {
-        typename std::iterator_traits<_InputIterator>::value_type __init = *__first;
+        typename iterator_traits<_InputIterator>::value_type __init = *__first;
         *__result++ = __init;
         if (++__first != __last)
             return _VSTD::inclusive_scan(__first, __last, __result, __b, __init);
@@ -391,7 +391,7 @@
 _OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last,
                                _OutputIterator __result)
 {
-    return _VSTD::inclusive_scan(__first, __last, __result, std::plus<>());
+    return _VSTD::inclusive_scan(__first, __last, __result, _VSTD::plus<>());
 }
 
 template <class _InputIterator, class _OutputIterator, class _Tp,
@@ -437,7 +437,7 @@
                                _OutputIterator __result, _BinaryOp __b, _UnaryOp __u)
 {
     if (__first != __last) {
-        typename std::iterator_traits<_InputIterator>::value_type __init = __u(*__first);
+        typename iterator_traits<_InputIterator>::value_type __init = __u(*__first);
         *__result++ = __init;
         if (++__first != __last)
             return _VSTD::transform_inclusive_scan(__first, __last, __result, __b, __u, __init);
@@ -576,8 +576,8 @@
 midpoint(_Tp __a, _Tp __b) noexcept
 _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
-    using _Up = std::make_unsigned_t<_Tp>;
-    constexpr _Up __bitshift = std::numeric_limits<_Up>::digits - 1;
+    using _Up = make_unsigned_t<_Tp>;
+    constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
 
     _Up __diff = _Up(__b) - _Up(__a);
     _Up __sign_bit = __b < __a;