We already defined the traits in <type_traits> as now required by LWG
3348, but the macro was missing. This adds it.

	* include/std/type_traits (__cpp_lib_unwrap_ref): Define (LWG 3348).
	* include/std/version (__cpp_lib_unwrap_ref): Likewise.
	* testsuite/20_util/unwrap_reference/1.cc: Check macro.
	* testsuite/20_util/unwrap_reference/3.cc: New test.