[hwasan] Fix lock contention on thread creation.

Do not hold the free/live thread list lock longer than necessary.
This change speeds up the following benchmark 10x.

constexpr int kTopThreads = 50;
constexpr int kChildThreads = 20;
constexpr int kChildIterations = 8;

void Thread() {
  for (int i = 0; i < kChildIterations; ++i) {
    std::vector<std::thread> threads;
    for (int i = 0; i < kChildThreads; ++i)
      threads.emplace_back([](){});
    for (auto& t : threads)
      t.join();
  }
}

int main() {
  std::vector<std::thread> threads;
  for (int i = 0; i < kTopThreads; ++i)
    threads.emplace_back(Thread);
  for (auto& t : threads)
    t.join();
}

Differential Revision: https://reviews.llvm.org/D100348