[ConfigImpact] Fix a bug on analyzing fields of configs

Summary:
When a field is assigned by a value,

```
_.field = exp;
```

it should collect the field when the abstract config value of `exp` is non-bottom, rather than
non-top.

Reviewed By: ezgicicek

Differential Revision: D27766188

fbshipit-source-id: a0b1f2c28

infer/src/cost/ConfigImpactAnalysis.ml

@@ -294,7 +294,7 @@ module Dom = struct
 
   let store_field fn id ({mem; config_fields} as astate) =
     let {Val.config} = Mem.lookup (Loc.of_id id) mem in
-    if ConfigLifted.is_top config then astate
+    if ConfigLifted.is_bottom config then astate
     else {astate with config_fields= Fields.add fn config_fields}