LWG 2993 reference_wrapper<T> conversion from T&&

LWG 2993 reference_wrapper<T> conversion from T&&

	* doc/xml/manual/intro.xml: Document LWG 2993 change.
	* include/bits/refwrap.h (reference_wrapper(_Tp&)): Remove.
	(reference_wrapper(_Tp&&)): Remove.
	(reference_wrapper<_Up>(_Up&&)): Define new constructor as constrained
	template.
	(reference_wrapper): Add deduction guide.
	* testsuite/20_util/reference_wrapper/deduction.cc: New.
	* testsuite/20_util/reference_wrapper/lwg2993.cc: New.

From-SVN: r261632