pass environment when invoking llvm-config from lit.cfg

Use the same environment when invoking llvm-config from lit.cfg as
will be used when running tests, so that ASAN_OPTIONS, INCLUDE, etc.
are present.

llvm-svn: 218403

test/lit.cfg

@@ -313,7 +313,8 @@ if have_ld_plugin_support():
 try:
     llvm_config_cmd = subprocess.Popen(
         [os.path.join(llvm_tools_dir, 'llvm-config'), '--assertion-mode'],
-        stdout = subprocess.PIPE)
+        stdout = subprocess.PIPE,
+        env=config.environment)
 except OSError:
     print("Could not find llvm-config in " + llvm_tools_dir)
     exit(42)