* 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
