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.
From-SVN: r268586
