Pass -swift-version X for %target-swift-frontend

Author: tkremenek

test/lit.cfg

@@ -709,8 +709,8 @@ if run_vendor == 'apple':
         "%s ld -L%s" %
         (xcrun_prefix, os.path.join(test_resource_dir, config.target_sdk_name)))
     config.target_swift_frontend = (
-        "%s -frontend %s -sdk %s %s" %
-        (config.swiftc, target_options, config.variant_sdk,
+        "%s -frontend -swift-version %s %s -sdk %s %s" %
+        (config.swiftc, swift_version, target_options, config.variant_sdk,
          config.swift_test_options))
     subst_target_swift_frontend_mock_sdk = (
         "%s -frontend %s -sdk %s %s" %