* doc/xml/manual/status_cxx2017.xml: Update status of not_fn.
	* doc/html/*: Regenerate.
	* include/experimental/functional (_Not_fn, not_fn): Match C++17
	semantics.
	* include/std/functional (_Not_fn, not_fn): Define for C++17.
	* testsuite/20_util/not_fn/1.cc: New.
	* testsuite/experimental/functional/not_fn.cc: Test abstract class.
	Remove test for volatile-qualified wrapper.

From-SVN: r239623