Migrate Java code to separate QL repo.

java/ql/src/Performance/ConcatenationInLoops.java

@@ -0,0 +1,18 @@
+public class ConcatenationInLoops {
+	public static void main(String[] args) {
+		Random r = new Random(123);
+		long start = System.currentTimeMillis();
+		String s = "";
+		for (int i = 0; i < 65536; i++)
+			s += r.nextInt(2);
+		System.out.println(System.currentTimeMillis() - start);  // This prints roughly 4500.
+
+		r = new Random(123);
+		start = System.currentTimeMillis();
+		StringBuilder sb = new StringBuilder();
+		for (int i = 0; i < 65536; i++)
+			sb.append(r.nextInt(2));
+		s = sb.toString();
+		System.out.println(System.currentTimeMillis() - start);  // This prints 5.
+	}
+}