Fix bug in lit.cfg where platform_module_dir would be a static string. This appears to have been harmless, but was still wrong.

Author: gwynne

test/lit.cfg

@@ -1325,7 +1325,7 @@ if hasattr(config, 'target_swift_autolink_extract'):
 config.substitutions.append(('%target-swift-modulewrap',
                              config.target_swift_modulewrap))
 
-platform_module_dir = make_path(test_resource_dir, '%target-sdk-name', run_cpu)
+platform_module_dir = make_path(test_resource_dir, config.target_sdk_name, run_cpu)
 lit_config.note('Using platform module dir: ' + platform_module_dir)
 if test_sdk_overlay_dir is not None:
     platform_sdk_overlay_dir = make_path(test_sdk_overlay_dir, run_cpu)