The P0318R1 paper added to the C++2a draft recently was not the latest
version of the paper, and should have included these changes. These
changes will be made to the working draft via a Defect Report, so I'm
applying them to libstdc++ now.

	* include/std/type_traits (unwrap_reference_t): Define for C++2a.
	(unwrap_ref_decay): Remove inheritance from unwrap_reference.
	* testsuite/20_util/unwrap_reference/1.cc: Adjust test to use alias.

From-SVN: r270506