* doc/xml/manual/status_cxx2017.xml: Document std::byte support.
	* include/c_global/cstddef (std::byte): Define for C++17.
	* testsuite/18_support/byte/global_neg.cc: New test.
	* testsuite/18_support/byte/ops.cc: New test.
	* testsuite/18_support/byte/requirements.cc: New test.

From-SVN: r246005