[Bug libstdc++/88996] New: Implement P0439R0 - Make std::memory_order a scoped enumeration.

[emsr at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88996

            Bug ID: 88996
           Summary: Implement P0439R0 - Make std::memory_order a scoped
                    enumeration.
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: emsr at gcc dot gnu.org
  Target Milestone: ---

Basically make memory_order an enum class and make constexpr aliases to teh old
names.
Without breaking ABI.