Fix running tests on macOS with python3

[arichardson]

Summary

The result of subprocess.check_output() is bytes in python3 which we need
to convert to str(). Simplify this by using the executeCommand() helper.

[arichardson]

utils/libcxx/test/target_info.py

133

I will remove this debug statement before committing.

[arichardson]

remove debug message

utils/libcxx/test/target_info.py

 import importlib
 import locale
 import os
 import platform
 import re
 import subprocess
 import sys
 
+from libcxx.util import executeCommand
+
 class DefaultTargetInfo(object):
     def __init__(self, full_config):
         self.full_config = full_config
 
     def platform(self):
         return sys.platform.lower().strip()
 
     def add_locale_features(self, features):
def add_locale_features(self, features):
         add_common_locales(features, self.full_config.lit_config)
 
     def add_cxx_compile_flags(self, flags):
         if self.full_config.use_deployment:
             _, name, _ = self.full_config.config.deployment
             cmd = ['xcrun', '--sdk', name, '--show-sdk-path']
         else:
             cmd = ['xcrun', '--show-sdk-path']
-        try:
-            out = subprocess.check_output(cmd).strip()
-            res = 0
-        except OSError:
-            res = -1
-        if res == 0 and out:
-            sdk_path = out
+        out, err, exit_code = executeCommand(cmd)
+        if exit_code != 0:
+            self.full_config.lit_config.warning("Could not determine macOS SDK path! stderr was " + err)
+        if exit_code == 0 and out:
+            sdk_path = out.strip()
             self.full_config.lit_config.note('using SDKROOT: %r' % sdk_path)
+            assert isinstance(sdk_path, str)
             flags += ["-isysroot", sdk_path]
 
     def add_cxx_link_flags(self, flags):
         flags += ['-lSystem']
 
     def configure_env(self, env):
         library_paths = []
         # Configure the library path for libc++