[lit] Fix running lit tests in unconfigured source dir

[mgorny]

Summary

Fix llvm_tools_dir attribute access not to fail when the variable is not
present. This directory is not really necessary to run lit tests,
and the code already accounts for it being None.

The reference was added in rL313407, and it breaks the stand-alone lit
package in Gentoo.

llvm/trunk/utils/lit/tests/lit.cfg

 except ImportError:
     lit_config.warning('Could not import psutil. Some tests will be skipped and'
                        ' the --timeout command line argument will not work.')
 
 if sys.platform.startswith('win') or sys.platform.startswith('cygwin'):
     config.available_features.add('windows')
 
 # Add llvm tools directory if this config is being loaded indirectly
-if config.llvm_tools_dir is not None:
+if getattr(config, 'llvm_tools_dir', None) is not None:
     path = os.path.pathsep.join((config.llvm_tools_dir, config.environment['PATH']))
     config.environment['PATH'] = path