[Support/Parallel] Use lock_guard which has less overhead than unique_lock.

[MaskRay]

Summary

unique_lock has the overhead of tracking ownership status and the owner.

[grimar]

std::condition_variable::wait requires unique_lock, but in other methods lock_guard should be enough,
I guess unique_lock is now there simply because of code copy-paste. So this change is LGTM.
Please wait for another opinion though, just in case.

[ruiu]

LGTM

llvm/trunk/include/llvm/Support/Parallel.h

class Latch {
   mutable std::mutex Mutex;
   mutable std::condition_variable Cond;
 
 public:
   explicit Latch(uint32_t Count = 0) : Count(Count) {}
   ~Latch() { sync(); }
 
   void inc() {
-    std::unique_lock<std::mutex> lock(Mutex);
+    std::lock_guard<std::mutex> lock(Mutex);
     ++Count;
   }
 
   void dec() {
-    std::unique_lock<std::mutex> lock(Mutex);
+    std::lock_guard<std::mutex> lock(Mutex);
     if (--Count == 0)
       Cond.notify_all();
   }
 
   void sync() const {
     std::unique_lock<std::mutex> lock(Mutex);
     Cond.wait(lock, [&] { return Count == 0; });
   }