libstdc++: Fix common_type specializations for duration

My recent change to implement P0548 ("common_type and duration") was not
correct. The result of common_type_t<duration<R,P>, duration<R,P>>
should be duration<common_type_t<R>, P::type>, not duration<R, P::type>.
The common_type specialization for two different duration types was
correct, but the specializations for a single duration type (which only
exist to optimize compilation time) were wrong.

This fixes the partial specializations of common_type for a single
duration type, and also the return types of duration::operator+ and
duration::operator- which are supposed to use common_type_t<duration>.

libstdc++-v3/ChangeLog:

	* include/std/chrono (common_type): Fix partial specializations
	for a single duration type to use the common_type of the rep.
	(duration::operator+, duration::operator-): Fix return types
	to also use the common_type of the rep.
	* testsuite/20_util/duration/requirements/reduced_period.cc:
	Check duration using a rep that has common_type specialized.

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