Dotty 0.20.0-RC1: Cannot write a given alias for a function

## minimized code
```Scala
given (Int => Int) = identity[Int]
```
Produces three errors:
end of statement expected but '(' found
';' expected, but identifier found
anonymous instance must implement a type or have at least one extension method

## expectation
The code above should work, just like the below does:

```scala
type IntFunc = Int => Int
given IntFunc = identity[Int]
```

