type_traits (common_type): Implement LWG 2141.

2012-10-24  Daniel Krugler  <daniel.kruegler@gmail.com>

	* include/std/type_traits (common_type): Implement LWG 2141.
	* testsuite/20_util/duration/requirements/sfinae_friendly_1.cc:
	Update.
	* testsuite/20_util/common_type/requirements/typedefs-1.cc: Likewise.
	* testsuite/20_util/common_type/requirements/sfinae_friendly_1.cc:
	Likewise.
	* testsuite/20_util/common_type/requirements/sfinae_friendly_2.cc:
	Likewise.
	* testsuite/20_util/common_type/requirements/typedefs-2.cc: Likewise.

From-SVN: r192780