`<chrono>`: Consider performing stricter checks in `is_clock_v`

[frederick-vs-ja]

Currently, MSVC STL only performs basic checks in std::chrono::is_clock(_v) as required by [time.traits.is.clock]/1.

STL/stl/inc/__msvc_chrono.hpp

Lines 62 to 70 in 1118f37

	_EXPORT_STD template <class _Clock>
	_NO_SPECIALIZATIONS_OF_VARIABLE_TEMPLATES constexpr bool is_clock_v = requires {
	typename _Clock::rep;
	typename _Clock::period;
	typename _Clock::duration;
	typename _Clock::time_point;
	_Clock::is_steady;
	_Clock::now();
	};

Some other implementations, like libstdc++, perform stricter checks. It might be better for MSVC STL to do so. Note that all requirements in the "Return type" column of [tab:time.clock], except for "a class emulating an arithmetic type" (see also #1909), seem statically checkable.

We may also want to perform equivalent checks (for multi-threading components) in old modes, although this requires rewriting in legacy SFINAE, which is still implementable.

STL/stl/inc/__msvc_chrono.hpp

Lines 77 to 84 in 1118f37

	template <class _Clock, class = void>
	constexpr bool _Is_clock_v = false;
	template <class _Clock>
	constexpr bool
	_Is_clock_v<_Clock, void_t<typename _Clock::rep, typename _Clock::period, typename _Clock::duration,
	typename _Clock::time_point, decltype(_Clock::is_steady), decltype(_Clock::now())>> =
	true;

[StephanTLavavej]

I'm significantly less eager to backport such checks to the pre-C++20 SFINAE implementation if they aren't required by the Standard.