* doc/xml/manual/intro.xml: Document DR 2296 status.
	* doc/xml/manual/status_cxx2017.xml: Update status.
	* include/bits/move.h (__addressof): Add _GLIBCXX_CONSTEXPR and
	call __builtin_addressof.
	(addressof): Add _GLIBCXX17_CONSTEXPR.
	* testsuite/20_util/addressof/requirements/constexpr.cc: New test.
	* testsuite/20_util/forward/c_neg.cc: Adjust dg-error lineno.
	* testsuite/20_util/forward/f_neg.cc: Likewise.

From-SVN: r240929