[libcxx][lit] Fix running testsuite with python2.7 after 9020d28688492c437abb648b6ab69baeba523219

[arichardson]

Summary

Python 2.7 fails with TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
if you pass None as the prefix argument to NamedTemporaryFile.

[bjope]

I have no clue if this has any negative impact from the https://reviews.llvm.org/D84040 point-of-view, etc. Maybe @ldionne can take a quick look at this as well (pre/post commit).

But I did confirm that this would make our downstream build bots happy again, when reporting the problem in D84040.
So LGTM, as it at least make things build again, with lit tests passing.

libcxx/utils/libcxx/test/dsl.py

def _executeScriptInternal(test, commands):
   for d in (execDir, os.path.dirname(tmpBase)):
     if not os.path.exists(d):
       os.makedirs(d)
   res = lit.TestRunner.executeScriptInternal(test, litConfig, tmpBase, parsedCommands, execDir)
   if isinstance(res, lit.Test.Result):
     res = ('', '', 127, None)
   return res
 
-def _makeConfigTest(config, testPrefix=None):
+def _makeConfigTest(config, testPrefix=''):
   sourceRoot = os.path.join(config.test_exec_root, '__config_src__')
   execRoot = os.path.join(config.test_exec_root, '__config_exec__')
   suite = lit.Test.TestSuite('__config__', sourceRoot, execRoot, config)
   if not os.path.exists(sourceRoot):
     os.makedirs(sourceRoot)
   tmp = tempfile.NamedTemporaryFile(dir=sourceRoot, delete=False, suffix='.cpp',
                                     prefix=testPrefix)
   tmp.close()
with _makeConfigTest(config) as test:
     with open(test.getSourcePath(), 'w') as sourceFile:
       sourceFile.write(source)
     out, err, exitCode, timeoutInfo = _executeScriptInternal(test, [
       "%{cxx} %s %{flags} %{compile_flags} %{link_flags} -o %t.exe"
     ])
     _executeScriptInternal(test, ['rm %t.exe'])
     return exitCode == 0
 
-def programOutput(config, program, args=[], testPrefix=None):
+def programOutput(config, program, args=[], testPrefix=''):
   """
   Compiles a program for the test target, run it on the test target and return
   the output.
 
   If the program fails to compile or run, None is returned instead. Note that
   execution of the program is done through the %{exec} substitution, which means
   that the program may be run on a remote host depending on what %{exec} does.
   """