Disallow unbound wildcard types like "val x: (_ <: Any) = 1"

[smarter]

If you write:

val x: (_ <: Any) = 1

In scalac you get:

error: unbound wildcard type

But in dotty we get (after the compiler stops desperately searching for a matching implicit view for several seconds):

error: type mismatch:
 found   : Int(1)
 required:

Labelling this issue as Parser because that's where the error is emitted in scalac but I don't know if we wish to reimplement that logic in there or do something later.