[libcxx] [test] Propagate host environment to libc++ test suite

[mgorny]

Summary

Propagate the complete host environment to the tests run via the new
testconfig. This ensures that all envvars needed e.g. for the compiler
to work correctly are present. This mimics the behavior explicitly
implemented in the legacy config.

https://github.com/llvm/llvm-project/issues/56816

[mgorny]

Thanks!

libcxx/test/libcxx/selftest/dsl/dsl.sh.py

def setUp(self):
             valgrindArgs=[],
             noExecute=False,
             debug=False,
             isWindows=platform.system() == 'Windows',
             order='smart',
             params={})
 
         self.config = lit.TestingConfig.TestingConfig.fromdefaults(self.litConfig)
+        self.config.environment = dict(os.environ)
         self.config.test_source_root = SOURCE_ROOT
         self.config.test_exec_root = EXEC_PATH
         self.config.recursiveExpansionLimit = 10
         self.config.substitutions = copy.deepcopy(SUBSTITUTIONS)
 
     def getSubstitution(self, substitution):
         """
         Return a given substitution from the TestingConfig. It is an error if

libcxx/utils/libcxx/test/newconfig.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 os
+
+
 def _getSubstitution(substitution, config):
   for (orig, replacement) in config.substitutions:
     if orig == substitution:
       return replacement
   raise ValueError('Substitution {} is not in the config.'.format(substitution))
 
 def configure(parameters, features, config, lit_config):
   note = lambda s: lit_config.note("({}) {}".format(config.name, s))
+  config.environment = dict(os.environ)
 
   # Apply the actions supplied by parameters to the configuration first, since
   # parameters are things that we request explicitly and which might influence
   # what features are implicitly made available next.
   for param in parameters:
     actions = param.getActions(config, lit_config.params)
     for action in actions:
       action.applyTo(config)