[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/array b/include/array
index da2ae2e..f4011c2 100644
--- a/include/array
+++ b/include/array
@@ -142,8 +142,8 @@
     typedef const value_type*                     const_pointer;
     typedef size_t                                size_type;
     typedef ptrdiff_t                             difference_type;
-    typedef std::reverse_iterator<iterator>       reverse_iterator;
-    typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+    typedef _VSTD::reverse_iterator<iterator>       reverse_iterator;
+    typedef _VSTD::reverse_iterator<const_iterator> const_reverse_iterator;
 
     _Tp __elems_[_Size];
 
@@ -155,7 +155,7 @@
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void swap(array& __a) _NOEXCEPT_(__is_nothrow_swappable<_Tp>::value) {
-        std::swap_ranges(data(), data() + _Size, __a.data());
+        _VSTD::swap_ranges(data(), data() + _Size, __a.data());
     }
 
     // iterators:
@@ -245,8 +245,8 @@
     typedef const value_type*                     const_pointer;
     typedef size_t                                size_type;
     typedef ptrdiff_t                             difference_type;
-    typedef std::reverse_iterator<iterator>       reverse_iterator;
-    typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+    typedef _VSTD::reverse_iterator<iterator>       reverse_iterator;
+    typedef _VSTD::reverse_iterator<const_iterator> const_reverse_iterator;
 
     typedef typename conditional<is_const<_Tp>::value, const char,
                                 char>::type _CharType;