`<atomic>`: On x64, `atomic_ref::is_lock_free()` incorrectly returns `true` when it shouldn't

[StephanTLavavej]

Repros with VS 2022 17.11 Preview 2 x64:

C:\Temp>type meow.cpp

#include <atomic>
#include <print>
using namespace std;

struct Large {
    char str[100]{};
};

int main() {
    Large lg{};
    atomic_ref<Large> atom_ref{lg};
    println("atomic_ref<Large> is_lock_free(): {}", atom_ref.is_lock_free());
    atom_ref.store(Large{"cute fluffy kittens"});
    println("lg.str: {}", lg.str);
}

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /Zi /Fdmeow.pdb meow.cpp
meow.cpp

C:\Temp>meow
atomic_ref<Large> is_lock_free(): true
lg.str: cute fluffy kittens

atomic_ref<Large>::store() definitely takes a lock:

STL/stl/inc/atomic

Lines 630 to 635 in e36ee6c

	void store(const _TVal _Value, const memory_order _Order = memory_order_seq_cst) noexcept {
	// store with sequential consistency
	_Check_store_memory_order(_Order);
	_Guard _Lock{_Spinlock};
	_Storage = _Value;
	}

atomic_ref::is_lock_free() is bogus because:

STL/stl/inc/atomic

Lines 2367 to 2376 in e36ee6c

	_NODISCARD bool is_lock_free() const noexcept {
	#if _ATOMIC_HAS_DCAS
	return is_always_lock_free;
	#else // ^^^ _ATOMIC_HAS_DCAS / !_ATOMIC_HAS_DCAS vvv
	if constexpr (is_always_lock_free) {
	return true;
	} else {
	return __std_atomic_has_cmpxchg16b() != 0;
	}
	#endif // ^^^ !_ATOMIC_HAS_DCAS ^^^

This meant to say: "If we're always lock-free then true, otherwise we have to be potentially lock-free and actually have cmpxchg16b."

I have a fix.