2019-03-04  Edward Smith-Rowland  <3dw4rd@verizon.net>

	PR libstdc++/88996 Implement P0439R0
	Make std::memory_order a scoped enumeration.
	* include/bits/atomic_base.h: For C++20 make memory_order a scoped enum,
	add variables for the old enumerators.  Adjust calls.
	* testsuite/29_atomics/headers/atomic/types_std_c++2a.cc: New test.
	* testsuite/29_atomics/headers/atomic/types_std_c++2a_neg.cc: New test.

From-SVN: r269372