`<xmemory>`: `allocator::is_always_equal` is incorrectly removed together with members removed in C++20

[frederick-vs-ja]

Describe the bug
std::allocator<T>::is_always_equal is merely deprecated, but not removed via LWG-3170. MSVC STL incorrectly removes it together with members of std::allocator that are deprecated in C++17 and removed in C++20 (in #1585).

Command-line test case

C:\Temp>type repro.cpp
#include <memory>

int main()
{
    static_assert(std::allocator<int>::is_always_equal::value, "");
}

C:\Temp>cl /EHsc /W4 /std:c++latest .\repro.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.29.30138 版
版权所有(C) Microsoft Corporation。保留所有权利。

/std:c++latest 作为最新的 C++
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https://go.microsoft.com/fwlink/?linkid=2045807。

repro.cpp
.\repro.cpp(5): error C3083: “is_always_equal”:“::”左侧的符号必须是一种类型
.\repro.cpp(5): error C2039: "value": 不是 "std::allocator<int>" 的成员
E:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xmemory(905): note: 参见“std::allocator<int>”的声明
.\repro.cpp(5): error C2065: “value”: 未声明的标识符

Expected behavior
This program compiles.

STL version
Microsoft Visual Studio Community 2019
Version 16.11.8

[CaseyCarter]

Yes, I suspect we confused ourselves in #1501 when we reused _CXX17_DEPRECATE_OLD_ALLOCATOR_MEMBERS for allocator::is_always_equal despite that it was actually applied to a C++20 working draft. When we implemented the later proposal to "remove the allocator members deprecated in C++17", it was naturally included by virtue of using the same deprecation control macro.

Looks like we need to introduce a new _CXX20_DEPRECATE_MEOW (under _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS) macro for allocator::is_always_equal.