Existential spelling diagnostic missing for `catch`

This is an error:

```swift
protocol P {
  associatedtype X
}

func foo(_ x: Any) {
  switch x {
  case is P: // error: use of protocol 'P' as a type must be written 'any P'
    break
  default:
    break
  }
}
```

but this is currently allowed:

```swift
do {

} catch is P {}
```

We ought to raise the same error.