add more information to benchmark test failures

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@367319 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/libcxx/test/googlebenchmark.py b/utils/libcxx/test/googlebenchmark.py
index 6fe731e..e531cba 100644
--- a/utils/libcxx/test/googlebenchmark.py
+++ b/utils/libcxx/test/googlebenchmark.py
@@ -110,7 +110,7 @@
                    )
 
         if exitCode:
-            return lit.Test.FAIL, out + err
+            return lit.Test.FAIL, ('exit code: %d\n' % exitCode) + out + err
 
         passing_test_line = testName
         if passing_test_line not in out: