Decide on the syntax of given in patterns

[smarter]

Currently the syntax for given in patterns looks like:

  def f1[T](x: Ordering[T]) = (x, x) match {
    case (given y: Ordering[T], _) => new TreeSet[T]
  }
  def f4[T](x: Ordering[T]) = {
    val xs = List(x, x, x)
    for given _: Ordering[T] <- xs
    yield new TreeSet[T]
  }

Which is different from the expression syntax where we write given Type or given name as Type, aligning ourselves would be nice, although the as syntax isn't so useful since we can use pattern bindings name @ given Type or name as given Type after #9837, so we could support only the given Type syntax.

[odersky]

In the Dotty meeting the consensus was to keep only the anonymous given Type syntax. If a name is desired, it can be added with a as binder as in other patterns. I.e. the example would be

def f1[T](x: Ordering[T]) = (x, x) match {
    case (given Ordering[T], _) => new TreeSet[T]
  }
  def f4[T](x: Ordering[T]) = {
    val xs = List(x, x, x)
    for given Ordering[T] <- xs
    yield new TreeSet[T]
    for x as given Ordering[T] <- xs
    yield new TreeSet[T]
  }