[Perf-Helper] Add logging for dtrace commands

Logging the dtrace command into the top of the dtrace log is useful when debugging why the order file generation is flaky.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@277234 91177308-0d34-0410-b5e6-96231b3b80d8

utils/perf-training/perf-helper.py

@@ -102,6 +102,7 @@ def dtrace(args):
   start_time = time.time()
 
   with open("%d.dtrace" % os.getpid(), "w") as f:
+    f.write("### Command: %s" % dtrace_args)
     subprocess.check_call(dtrace_args, stdout=f, stderr=subprocess.PIPE)
 
   elapsed = time.time() - start_time