Fix a data race found by tsan.

Reads from `Live` and writes to `OutputOff` in the following code race
even though they are logically independent because they are bitfields
sharing the same word.

  for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I) {
    if (!Sec->Pieces[I].Live)
      continue;
    CachedHashStringRef Str = Sec->getData(I);
    size_t ShardId = getShardId(Str.hash());
    if ((ShardId & (Concurrency - 1)) == ThreadId)
      Sec->Pieces[I].OutputOff = Shards[ShardId].add(Str);
  }

llvm-svn: 314711