[perf-test] Add in the Benchmark_DTrace driver.

This and the associated *.d file can be used to determine dynamic
retain/release counts over the perf test suite.

benchmark/scripts/perf_test_driver/swift_stats.d

@@ -0,0 +1,27 @@
+/*===--- swift_stats.d ----------------------------------------------------===//
+ *
+ * This source file is part of the Swift.org open source project
+ *
+ * Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+ * Licensed under Apache License v2.0 with Runtime Library Exception
+ *
+ * See http://swift.org/LICENSE.txt for license information
+ * See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+ *
+ *===----------------------------------------------------------------------===*/
+
+pid$target:*:swift_retain:entry
+{
+        @counts[probefunc] = count();
+}
+
+pid$target:*:swift_release:entry
+{
+        @counts[probefunc] = count();
+}
+
+END
+{
+        printf("\nDTRACE RESULTS\n");
+        printa("%s,%@u\n", @counts)
+}