PR libstdc++/89102 fix common_type<> and common_type<T> specializations

PR libstdc++/89102 fix common_type<> and common_type<T> specializations

This is a partial implementation of the revised std::common_type rules
from P0435R1.

	PR libstdc++/89102 (partial)
	* include/std/type_traits (common_type<>): Define.
	(common_type<T>): Derive from common_type<T, T>.
	* testsuite/20_util/common_type/requirements/explicit_instantiation.cc:
	Test zero-length template argument list.
	* testsuite/20_util/common_type/requirements/sfinae_friendly_1.cc:
	Test additional single argument cases.
	* testsuite/20_util/common_type/requirements/sfinae_friendly_2.cc:
	Adjust expected error.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@268586 138bc75d-0d04-0410-961f-82ee72b054a4