[lld][Core] Fix latch synchronization bug.

We need to acquire a lock before signal a condition.
Otherwise threads waiting on a condition variable can
miss a signal.

Consider two threads: Thread A executing dec() and thread
B executing sync(). The initial value of _count is 1. If
these two threads are interleaved in the following order,
thread B misses the signal sent by thread A, because at the
time thread A sends a signal, B is not waiting for it.

  Thread A                 |   Thread B
                           |   std::unique_lock<std::mutex> lock(_condMut);
                           |   while (!(_count == 0)) {
  if (--_count == 0)       |
    _cond_notify_all()     |
                           |     _cond.wait();
                           |   }

Note that "wait(lock, pred)" is equivalent to "while(!pred())
wait(lock)", so I expanded it in the above example.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D764

llvm-svn: 181484