[libc++] Change memory_order to an enum class

This implements P0439R0.

Thanks to Zoe Carver for the patch.
Differential Revision: https://reviews.llvm.org/D58201

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355403 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed