PR libstdc++/81468
	* include/std/chrono (__enable_if_is_duration)
	(__disable_if_is_duration): New alias templates to simplify SFINAE.
	(duration_cast, floor, ceil): Use __enable_if_is_duration.
	(duration::__is_float, duration::__is_harmonic): New alias templates
	to simplify SFINAE.
	(duration::duration(const _Rep2&)): Use _Require, __is_float and
	__is_harmonic.
	(duration::duration(const duration<_Rep2, _Period2>&)): Likewise.
	(__common_rep_type): Remove, replace with ...
	(__common_rep_t): New alias template.
	(operator*, operator/, operator%): Use __common_rep_t and
	__disable_if_is_duration.
	(time_point::time_point(const time_point<clock, _Dur2>&)): Add missing
	constraint from LWG DR 1177.
	* testsuite/20_util/duration/cons/dr1177.cc: New.
	* testsuite/20_util/duration/literals/range.cc: Update dg-error line.
	* testsuite/20_util/duration/requirements/typedefs_neg1.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg3.cc: Likewise.
	* testsuite/20_util/time_point/cons/81468.cc: New.

From-SVN: r252085