[libc++] Un-deprecate and un-remove some members of std::allocator

This implements the part of P0619R4 related to the default allocator.
This is incredibly important, since otherwise there is an ABI break
between C++17 and C++20 w.r.t. the default allocator's size_type on
platforms where std::size_t is not the same as std::make_unsigned<std::ptrdiff_t>.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 316d336dca7f64f048507f155166daa3821db957
diff --git a/include/memory b/include/memory
index 54176f4..ebb0a72 100644
--- a/include/memory
+++ b/include/memory
@@ -115,8 +115,8 @@
 class allocator
 {
 public:
-    typedef size_t    size_type;                         // deprecated in C++17, removed in C++20
-    typedef ptrdiff_t difference_type;                   // deprecated in C++17, removed in C++20
+    typedef size_t    size_type;
+    typedef ptrdiff_t difference_type;
     typedef T*        pointer;                           // deprecated in C++17, removed in C++20
     typedef const T*  const_pointer;                     // deprecated in C++17, removed in C++20
     typedef typename add_lvalue_reference<T>::type
@@ -1748,9 +1748,9 @@
 class _LIBCPP_TEMPLATE_VIS allocator
 {
 public:
+    typedef size_t     size_type;
+    typedef ptrdiff_t  difference_type;
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
-    _LIBCPP_DEPRECATED_IN_CXX17 typedef size_t     size_type;
-    _LIBCPP_DEPRECATED_IN_CXX17 typedef ptrdiff_t  difference_type;
     _LIBCPP_DEPRECATED_IN_CXX17 typedef _Tp*       pointer;
     _LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp* const_pointer;
     _LIBCPP_DEPRECATED_IN_CXX17 typedef _Tp&       reference;
@@ -1816,9 +1816,9 @@
 class _LIBCPP_TEMPLATE_VIS allocator<const _Tp>
 {
 public:
+    typedef size_t     size_type;
+    typedef ptrdiff_t  difference_type;
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
-    _LIBCPP_DEPRECATED_IN_CXX17 typedef size_t     size_type;
-    _LIBCPP_DEPRECATED_IN_CXX17 typedef ptrdiff_t  difference_type;
     _LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp* pointer;
     _LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp* const_pointer;
     _LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp& reference;