[libc++] Do not specify the underlying type of memory_order

Summary:
This breaks ABI for folks using -fshort-enums, and does not really buy
us anything.

http://llvm.org/PR40977

Reviewers: mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits, zoecarver

Differential Revision: https://reviews.llvm.org/D59029

llvm-svn: 355521