[libcxx][dsl] Fix mutable default argument warning

[arichardson]

Summary

This is flagged by PyCharm and can cause subtle bugs. While changing this
also re-sort the imports and add missing ones.

[ldionne]

Thanks! You have commit access, right?

[arichardson]

Yes, will commit shortly. Thanks for reviewing!

libcxx/utils/libcxx/test/dsl.py

 #===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===----------------------------------------------------------------------===##
 
-import libcxx.test.format
-import lit
-import lit.util
 import os
 import pipes
 import platform
 import re
 import tempfile
 
+import libcxx.test.format
+import lit
+import lit.LitConfig
+import lit.Test
+import lit.TestRunner
+import lit.util
+
+
 def _memoize(f):
   cache = dict()
   def memoized(x):
     if x not in cache:
       cache[x] = f(x)
     return cache[x]
   return memoized
 
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=''):
+def programOutput(config, program, args=None, 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.
   """
+  if args is None:
+    args = []
   with _makeConfigTest(config, testPrefix=testPrefix) as test:
     with open(test.getSourcePath(), 'w') as source:
       source.write(program)
     try:
       _, _, exitCode, _ = _executeScriptInternal(test, [
         "%{cxx} %s %{flags} %{compile_flags} %{link_flags} -o %t.exe",
       ])
       if exitCode != 0: