SI-4827 Corrected positions assigned to constructor's default arg

* Default arguments are always retained on the <init> method (i.e.,
  the class' constructor). Therefore, when the <init> parameters are
  created, we need to use `duplicateAndKeepPositions` to make sure that
  if a default argument is present, its (potentially opaque) position is
  retained as well. This is necessary because when parameter accessors
  (i.e., `fieldDefs`) are created, default arguments are discared (if
  present) - see change in TreeGen.scala

* When constructing the `fieldDefs`, it is important to adapt their
  range position to avoid overlappings with the positions of default
  arguments. It is worth noting that updating the field's end position
  to `vd.rhs.pos.start` would be incorrect, because `askTypeAt(pos)`
  could return the incorrect tree when the position is equal to
  `vd.rhs.pos.start` (because two nodes including that point position
  would exist in the tree, and `CompilerControl.locateTree(pos)` would
  return the first tree that includes the passed `pos`).  This should
  motivate why `vd.tpt.pos.end` is used instead, as it ensures that the
  range position assigned to the field does not overlap (at all) with
  the position assigned to the default arguments - see changed in
  TreeGen.scala.

* If the synthetic constructor contains trees with an opaque range
  position (see point above), it must have a transparent position.
  This can only happen if the constructor's parameters' positions are
  considered, which is why we are now passing `vparamss1` to
  `wrappingPos` - see change in TreeGen.scala.

* The derived primary constructor should have a transparent position
  as it may contain trees with an opaque range position. Hence, the
  `primaryCtor` position is considered for computing the position of the
  derived constructor - see change in Typers.scala.

Finally, below follows the printing of the tree for test t4287, which
you should compare with the one attached with the previous commit
message:

```
[[syntax trees at end of                     typer]] // Foo.scala
[0:63]package [0:0]<empty> {
  [0:37]class Baz extends [9:37][39]scala.AnyRef {
    [10:20]<paramaccessor> private[this] val f: [14]Int = _;
    [14]<stable> <accessor> <paramaccessor> def f: [14]Int = [14][14]Baz.this.f;
    <10:31>def <init>(<10:31>f: [17]<type: [17]scala.Int> = [23:31]B.a): [9]Baz = <10:31>{
      <10:31><10:31><10:31>Baz.super.<init>();
      <10:31>()
    }
  };
  [6]<synthetic> object Baz extends [6][6]AnyRef {
    [6]def <init>(): [9]Baz.type = [6]{
      [6][6][6]Baz.super.<init>();
      [9]()
    };
    [14]<synthetic> def <init>$default$1: [14]Int = [30]B.a
  };
  [39:63]object B extends [48:63][63]scala.AnyRef {
    [63]def <init>(): [48]B.type = [63]{
      [63][63][63]B.super.<init>();
      [48]()
    };
    [52:61]private[this] val a: [56]Int = [60:61]2;
    [56]<stable> <accessor> def a: [56]Int = [56][56]B.this.a
  }
}
```

You should notice that the default arg of `Baz` constructor now has a
range position. And that explains why the associated test now returns
the right tree when asking hyperlinking at the location of the default
argument.