<compare>: `std::_Literal_zero` has an impact on real code.

Describe the bug

Our type std::_Literal_zero has an impact on real code.

From the comment: https://github.com/microsoft/STL/pull/4332#issuecomment-1919524313

Command-line test case

#include <compare>

template<typename T, typename U>
constexpr bool not_equal(T lhs, U rhs) 
requires(requires(T lhs, U rhs) { lhs != rhs; })
{
        return lhs != rhs;
}

template<typename T, typename U>
constexpr bool not_equal(T lhs, U rhs) 
requires(!requires(T lhs, U rhs) { lhs != rhs; })
{
        return false;
}

int main() {
    constexpr int int_zero = 0;
    constexpr unsigned int unsigned_zero = 0;
    constexpr auto ordering = 1 <=> 1;
    //constexpr bool b = not_equal(ordering, unsigned_zero);
    constexpr bool b = not_equal(ordering, int_zero);
    return b == false;
}


C:\Temp>cl /EHsc /W4 /WX /std:c++latest .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.39.33321 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

/std:c++latest is provided as a preview of language features from the latest C++
working draft, and we're eager to hear about bugs and suggestions for improvements.
However, note that these features are provided as-is without support, and subject
to changes or removal as the working draft evolves. See
https://go.microsoft.com/fwlink/?linkid=2045807 for details.

repro.cpp
.\repro.cpp(7): error C7595: 'std::_Literal_zero::_Literal_zero': call to immediate function is not a constant expression
.\repro.cpp(7): note: failure was caused by a read of a variable outside its lifetime
.\repro.cpp(7): note: see usage of 'rhs'
.\repro.cpp(7): note: the template instantiation context (the oldest one first) is
.\repro.cpp(22): note: see reference to function template instantiation 'bool not_equal<std::strong_ordering,int>(T,U)' being compiled
        with
        [
            T=std::strong_ordering,
            U=int
        ]

Expected behavior

Should compile

STL version

https://github.com/microsoft/STL/commit/202e3820d6390505855ce9a58a4bcee4f28feacd

Additional context

Other compiler libraries compile the code: https://godbolt.org/z/1vbYbq5b7

We added std::_Literal_zero because of this issue: https://github.com/microsoft/STL/pull/3581

Previously using _Literal_zero = decltype(nullptr); warned this code:

#include <compare>

auto b = 1 <=> 2 < 0;

int main() {}

clang-cl /std:c++20 -Wzero-as-null-pointer-constant main.cpp
main.cpp(3,20): warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
auto b = 1 <=> 2 < 0;
                   ^
                   nullptr
1 warning generated.

Start by compiling the supplied reproducer with the referenced STL commit and compare the behavior of the two not_equal overloads. Read the implementation introduced for std::_Literal_zero and the earlier change linked in the issue; done means the reproducer compiles while preserving the warning-related behavior described in the report.