libstdc++: Define and use variable templates in <chrono>

Thi defines a variable template for the internal __is_duration helper
trait, defines a new __is_time_point_v variable template (to be used in
a subsequent commit), and adds explicit specializations of the standard
chrono::treat_as_floating_point trait for common types.

A fast path is added to chrono::duration_cast for the no-op case where
no conversion is needed.

Finally, some SFINAE constraints are simplified by using the
__enable_if_t alias, or by using variable templates.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (__is_duration_v, __is_time_point_v):
	New variable templates.
	(duration_cast): Add simplified definition for noconv case.
	(treat_as_floating_point_v): Add explicit specializations.
	(duration::operator%=, floor, ceil, round): Simplify SFINAE
	constraints.

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