[F#] Generated resource source files show as squiggles in editor

<img width="947" alt="image" src="https://user-images.githubusercontent.com/667194/71981648-b70f6780-321a-11ea-9438-5833f3b7589a.png">

<img width="1016" alt="image" src="https://user-images.githubusercontent.com/667194/71981797-048bd480-321b-11ea-88ce-b2a737e0f190.png">

Project compiles just fine though.

> VS bug [#1049376](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1049376)