[tsan] Treat non-barrier OSAtomic* functions as barriers to avoid false positives

The non-barrier versions of OSAtomic* functions are semantically mo_relaxed, but the two variants (e.g. OSAtomicAdd32 and OSAtomicAdd32Barrier) are actually aliases of each other, and we cannot have different interceptors for them, because they're actually the same function. Thus, we have to stay conservative and treat the non-barrier versions as mo_acq_rel.

Differential Revision: http://reviews.llvm.org/D21733

llvm-svn: 273890