Add comparison between QDateTime::fromMSecsSinceEpoch

It is much faster to directly get the msecs in UTC rather then
first getting a local timestamp and converting it to UTC afterwards:

```
PASS   : BenchQDateTime::benchFromMSecsSinceEpoch()
RESULT : BenchQDateTime::benchFromMSecsSinceEpoch():
     0.0057 msecs per iteration (total: 94, iterations: 16384)
PASS   : BenchQDateTime::benchFromMSecsSinceEpochToUTC()
RESULT : BenchQDateTime::benchFromMSecsSinceEpochToUTC():
     0.0075 msecs per iteration (total: 62, iterations: 8192)
PASS   : BenchQDateTime::benchFromMSecsSinceEpochUTC()
RESULT : BenchQDateTime::benchFromMSecsSinceEpochUTC():
     0.000013 msecs per iteration (total: 57, iterations: 4194304)
```

bench_qdatetime/bench_qdatetime.cpp

@@ -64,6 +64,39 @@ private slots:
         }
     }
 
+    Q_NEVER_INLINE void benchFromMSecsSinceEpoch()
+    {
+        qint64 msecs = 0;
+        clobber();
+
+        QBENCHMARK {
+            auto a = QDateTime::fromMSecsSinceEpoch(msecs);
+            escape(&a);
+        }
+    }
+
+    Q_NEVER_INLINE void benchFromMSecsSinceEpochToUTC()
+    {
+        qint64 msecs = 0;
+        clobber();
+
+        QBENCHMARK {
+            auto a = QDateTime::fromMSecsSinceEpoch(msecs).toUTC();
+            escape(&a);
+        }
+    }
+
+    Q_NEVER_INLINE void benchFromMSecsSinceEpochUTC()
+    {
+        qint64 msecs = 0;
+        clobber();
+
+        QBENCHMARK {
+            auto a = QDateTime::fromMSecsSinceEpoch(msecs, Qt::UTC);
+            escape(&a);
+        }
+    }
+
     Q_NEVER_INLINE void benchQElapsedTimer()
     {
         QBENCHMARK {