`add C to the context` code action doesn't appear for instances

Consider the following:

```haskell
class Yo f where
yo :: f x -> Int

go :: M1 i c f x -> Int
go (M1 fx) = yo fx
```

This gives me an underline on the call to `yo`, complaining that there is no instance `Yo f`. It comes with a code action to insert it into the type.

Now consider the analogous:

```haskell
class Yo f where
yo :: f x -> Int

instance Yo (M1 _1 _2 f) where
yo (M1 fx) = yo fx
```

Same diagnostic, but this time there is no code action to fix the issue!