Ryan Scott authored Jun 07, 2023 and Marge Bot committed Jun 09, 2023

We were checking that TH-spliced data declarations do not look like this:

```hs
data D :: Type = MkD Int
```

But we were only doing so for `data` declarations' data constructors, not for
`newtype`s, `data instance`s, or `newtype instance`s. This patch factors out
the necessary validity checks into its own `cvtDataDefnCons` function and uses
it in all of the places where it needs to be.

Fixes #22559.