[v3] RFC: rename __calculate_memory_order

[Benjamin Kosnik]

>         * include/bits/atomic_base.h (__calculate_memory_order):
> Rename to... (__cmpexch_failure_order): This, and rewrite as
> constexpr function. (compare_exchange_strong, compare_exchange_weak):
> Use it.
>         * include/std/atomic (compare_exchange_strong,
> compare_exchange_weak): Likewise.
> 
> Tested x86_64-linux.

looks great to me. More constexpr, what's not to like?

-benjamin