Implement LWG-3865 Sorting a range of pairs

[fsb4000]

Fixes #3419

[fsb4000]

libc++ test has

// Needed by do_forwarding_test() since it compares pairs of different types.
template <class T1, class T2, class U1, class U2>
inline bool operator==(const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs) {
    return lhs.first == rhs.first && lhs.second == rhs.second;
}

and because of that we get

C:\Dev\STL\llvm-project\libcxx\test\std\utilities\tuple\tuple.tuple\tuple.apply\make_from_tuple.pass.cpp(75,23): error: use of overloaded operator '==' is ambiguous (with operand types 'const pair<std::decay_t<int &>, std::decay_t<const char *>>' (aka 'const pair<int, const char *>') and 'std::pair<int &, const char *>')
return value.args == tup
~~~~~~~~~~ ^ ~~~

[strega-nil-ms]

I feel comfortable unmarking decision needed; given that it'd be pretty difficult to get broken by these changes, I'm totally willing to treat this as a DR.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for implementing this significant usability improvement - one of my top annoyances from C++98! 😻 🎉 ✨