Related to #1749 - we should avoid using concepts when __cpp_lib_concepts isn't defined.

• Cleanup: When performing SFINAE in default template arguments, use class = void_t<decltype(EXPRESSION)> for consistency (and shorter mangled names, although that doesn't really matter). This is still different than the most common form of enable_if_t<CONDITION, int> = 0 SFINAE which handles multiple overloads with the same signature, but that should be ok.
• Cleanup: Update the comments about ICU/IANA. Thanks to @mattjohnsonpint for explaining what's happening here in <chrono>: Should divergence between ICU and IANA be reported? #1786.
• hh_mm_ss can static_assert _Is_duration_v.
  • This is consistent with time_point:

    STL/stl/inc/chrono

    Lines 198 to 206 in 6aed1a6

    	template <class _Clock, class _Duration = typename _Clock::duration>
    	class time_point { // represents a point in time
    	public:
    	using clock = _Clock;
    	using duration = _Duration;
    	using rep = typename _Duration::rep;
    	using period = typename _Duration::period;
    	static_assert(_Is_duration_v<_Duration>, "duration must be an instance of std::duration");

• Cleanup: The static_assert _Is_duration_v messages now say "mandates".
  • time_point was talking about std::duration which isn't a thing! 😹
• Reimplement clock_time_conversion Constraints/Mandates.
  • Thanks to @MattStephanson for help decoding the WP here (see [time.clock.cast.sys] and [time.clock.cast.utc] reuse "Duration" confusingly cplusplus/draft#4564).
  • This implements the Standardese more precisely - e.g. WG21-N4885 [time.clock.cast.sys]/1 "Constraints: SourceClock::to_sys(t) is well-formed." isn't concerned about the return type, only /2 "Mandates: SourceClock::to_sys(t) returns a sys_time<Duration>" is.
  • The part of the Mandates that says "where Duration is a valid chrono::duration specialization" is enforced by time_point itself (which we're going to instantiate while attempting to return it).
  • We need _SourceClock2 and _DestClock2 as an extra layer of indirection because these template parameters belong to the struct instead of the function.
• Rewrite clock_cast metaprogramming.
  • This uses @CaseyCarter's "choice" approach from ranges. Here's it's almost too high-powered (compared to just directly testing these things in clock_cast's function body), but not quite - we need to implement [time.clock.cast.fn]/1 "Constraints: At least one of the following clock time conversion expressions is well-formed" (which wasn't previously implemented as a constraint - instead it was a static_assert which would be a Mandates), so the "choice" tech lets us reuse that work.