libstdc++: Make std::chrono::duration use reduced ratio for period

This implements the changes from P0548 "common_type and duration". That
was a change for C++17, but as it corrects some issues introduced by DRs
I'm also treating it as a DR and changing it for all modes from C++11
up.

The main change is that duration<R,P>::period no longer denotes P, but
rather P::type, the reduced ratio. The unary operator+ and operator-
members of duration should now return a duration using that reduced
ratio.

The requirement that common_type<T>::type is the same type as
common_type<T, T>::type (rather than simply T) was already implemented
for PR 89102.

The standard says that duration::operator+() and duration::operator-()
should return common_type_t<duration>, but that seems unnecessarily
expensive to compute. This change just uses duration<rep, period> which
is the same type, so we don't need to instantiate common_type.

As an optimization, this also adds partial specializations of
common_type for two durations of the same type, a single duration, two
time_points of the same type, and a single time_point. These
specializations avoid instantiating other specializations of common_type
and one or both of __duration_common_type or __timepoint_common_type for
the cases where the answer is trivial to obtain.

libstdc++-v3/ChangeLog:

	* include/std/chrono (__duration_common_type): Ensure the
	reduced ratio is used. Remove unused partial specialization
	using __failure_type.
	(common_type): Pass reduced ratios to __duration_common_type.
	Add partial specializations for simple cases involving a single
	duration or time_point type.
	(duration::period): Use reduced ratio.
	(duration::operator+(), duration::operator-()): Return duration
	type using the reduced ratio.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc:
	Adjust expected errors.
	* testsuite/20_util/duration/requirements/reduced_period.cc: New test.

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