Implement C++17 rounding functions for std::chrono (P0092R1)

Implement C++17 rounding functions for std::chrono (P0092R1)

	* include/std/chrono (floor, ceil, round, abs): New for C++17.
	* testsuite/20_util/duration_cast/rounding.cc: New test.
	* testsuite/20_util/time_point_cast/rounding.cc: New test.
	* doc/xml/manual/status_cxx2017.xml: Update status table.
	* doc/html/manual/status.html: Regenerate.
	* testsuite/20_util/duration/requirements/typedefs_neg1.cc: Adjust
	dg-error lineno.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg3.cc: Likewise.
	* testsuite/20_util/duration/literals/range.cc: Likewise.

From-SVN: r239170