Reapply "NFC: utils/perf-training: Python 3 compatibility for lit.cfg"

This is in response to the recent move to Python 3.

This reverts commit 5f4426e517.

clang/utils/perf-training/lit.cfg

@@ -10,7 +10,7 @@ def getSysrootFlagsOnDarwin(config, lit_config):
     # default system root path.
     if 'darwin' in config.target_triple:
         try:
-            out = subprocess.check_output(['xcrun', '--show-sdk-path']).strip()
+            out = subprocess.check_output(['xcrun', '--show-sdk-path']).strip().decode()
             res = 0
         except OSError:
             res = -1