Merge pull request swiftlang#39168 from xymus/library-level-feature-flag

[Driver] Define the feature flag library-level

Author: xymus

lib/Option/features.json

@@ -8,6 +8,9 @@
     },
     {
       "name": "index-unit-output-path"
+    },
+    {
+      "name": "library-level"
     }
   ]
 }