Expand type synonyms in markNominalwip/T20999

`markNominal` is repsonsible for setting the roles of type variables
that appear underneath an `AppTy` to be nominal. However, `markNominal`
previously did not expand type synonyms, so in a data type like this:

```hs
data M f a = MkM (f (T a))

type T a = Int
```

The `a` in `M f a` would be marked nominal, even though `T a` would simply
expand to `Int`. The fix is simple: call `coreView` as appropriate in
`markNominal`. This is much like the fix for #14101, but in a different spot.

Fixes #20999.

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