[6.2][Option] Add a feature flag for -Isystem

swift-build needs a flag to know if -Isystem is available.

rdar://152540225

Author: ian-twilightcoder

lib/Option/features.json

@@ -47,6 +47,9 @@
     },
     {
        "name": "experimental-package-interface-load"
+    },
+    {
+       "name": "Isystem"
     }
   ]
 }