[Bug c/69104] invalid atomic memory order not diagnosed

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69104

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
Patch documenting -Winvalid-memory-model posted for review:
https://gcc.gnu.org/ml/gcc-patches/2016-01/msg00111.html