Sema: demote invalid atomic ordering message to warning.

Someone could write:
  if (0) {
    __c11_atomic_load(ptr, memory_order_release);
  }

or the equivalent, which is perfectly valid, so we shouldn't outright reject
invalid orderings on purely static grounds.

rdar://problem/16242991

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203564 91177308-0d34-0410-b5e6-96231b3b80d8