Make std::tuple_size<cv T> SFINAE-friendly (LWG 2770)

	* doc/xml/manual/intro.xml: Document LWG 2770 status. Remove entries
	for 2742 and 2748.
	* doc/html/*: Regenerate.
	* include/std/utility (__tuple_size_cv_impl): New helper to safely
	detect tuple_size<T>::value, as per LWG 2770.
	(tuple_size<cv T>): Adjust partial specializations to derive from
	__tuple_size_cv_impl.
	* testsuite/20_util/tuple/cv_tuple_size.cc: Test SFINAE-friendliness.

From-SVN: r242452