libstdc++: Invert relationship between std::is_clock and std::is_clock_v

libstdc++: Invert relationship between std::is_clock and std::is_clock_v

This redefines std::is_clock in terms of std::is_clock_v, instead of the
other way around. This avoids instantiatng the class template for code
that only uses the variable template.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (is_clock_v): Define to false.
	(is_clock_v<T>): Define partial specialization for true cases.
	(is_clock): Define in terms of is_clock_v.