For
`__atomic_compare_exchange{,_n}/__c11_atomic_compare_exchange_{strong,weak}`,
GCC checks both the success memory order and the failure memory order
under the default -Winvalid-memory-model ("memory model" is confusing
here and "memory order" is much more common in the atomic context).
* The failure memory order, if a constant, must be one of
relaxed/consume/acquire/seq_cst.
Clang checks just the success memory order under the default
-Watomic-memory-ordering. This patch checks the failure memory order.
fed564432c