Avoid triggering assert when program calls OSAtomicCompareAndSwapLong

A previous change brought the new, relaxed implementation of "on failure
memory ordering" for synchronization primitives in LLVM over to TSan
land [1].  It included the following assert:
```
// 31.7.2.18: "The failure argument shall not be memory_order_release
// nor memory_order_acq_rel". LLVM (2021-05) fallbacks to Monotonic
// (mo_relaxed) when those are used.
CHECK(IsLoadOrder(fmo));

static bool IsLoadOrder(morder mo) {
  return mo == mo_relaxed || mo == mo_consume
      || mo == mo_acquire || mo == mo_seq_cst;
}
```

A previous workaround for a false positive when using an old Darwin
synchronization API assumed this failure mode to be unused and passed a
dummy value [2].  We update this value to `mo_relaxed` which is also the
value used by the actual implementation to avoid triggering the assert.

[1] https://reviews.llvm.org/D99434
[2] https://reviews.llvm.org/D21733

rdar://78122243

Differential Revision: https://reviews.llvm.org/D105844