Use correct var name for element when handling arrays of type aliases

Summary:
Fixes an issue where, if a spec uses an array of elements where the element is a type alias, the generated code would use the wrong variable name.

An example of such a spec can be found in `NativeExceptionsManager.js`:
```
  +reportSoftException: (
    message: string,
    stack: Array<StackFrame>,
    exceptionId: number,
  ) => void;
```

The fix ensures the local variable name is passed through, ensuring that either p or itemValue_N is used when appropriate.

Changelog: [Internal]

Reviewed By: RSNara

Differential Revision: D24116429

fbshipit-source-id: f39b75adb604c751d70a284a11a7fa6649b1344d

packages/react-native-codegen/src/generators/modules/GenerateModuleObjCpp/header/serializeRegularStruct.js

@@ -195,8 +195,8 @@ function toObjCValue(
       );
 
       return !isRequired
-        ? `(p == nil ? folly::none : folly::make_optional(${namespacedStructName}(p)))`
-        : `${namespacedStructName}(p)`;
+        ? `(${value} == nil ? folly::none : folly::make_optional(${namespacedStructName}(${value})))`
+        : `${namespacedStructName}(${value})`;
     default:
       (typeAnnotation.type: empty);
       throw new Error(