Prevent `this` with transparent position to be indexed

This change was needed to successfully run all scala-refactoring
test based on a Scala compiler that contains a fix for
https://issues.scala-lang.org/browse/SI-4287 (PR for this ticket
will follow shortly).

The fix for the compiler mentioned above affects the positions
assigned to synthetic constructor Trees, and that was causing the
test `MoveClassTest.copyrightHeaderIsMovedAlong` to fail because
an additional (and unexpected) `import org.com.pkg.B` clause was
being inserted in the refactored source.
While debugging the failing test, I've compared the content of the
`CompilationUnitIndex` before and after the changes made in SI-4287,
and noticed that an additional reference for the `class B` symbol to
the `this` tree was being cached while traversing the
`B.super.<init>()` apply node, in `B` syntehtic constructor.

Which is why is thought that the `this` reference should be stored in
the index iff the position is an opaque range (i.e., the reference
existed in the original source), while it should be skipped if the
position is a transparent range.

That also made me wonder if, as a general rule, leaf trees with a non
opaque range position should be always skipped.

Below follows the result of printing the tree produced by the test
in `MoveClassTest.copyrightHeaderIsMovedAlong`, before and after fixing
SI-4287. It is worth noting that the positions of the trees in `B`
synthetic constructor have changed, causing the issue described in the
second paragraph of this commit.

Before fixing SI-4287
--------------------
```
[[syntax trees at end of                     typer]]
[21:82]package [29:36]org.com {
  [37:82]package [45:48]pkg {
    [50:57]abstract trait A extends [57:57][59]scala.AnyRef;
    [59:82]class B extends [76:82][82]scala.AnyRef {
      [77:81]<paramaccessor> private[this] val a: [77]org.com.pkg.A = _;
      [82]def <init>([77]a: [80]<type: [80]A>): [76]org.com.pkg.B = [82]{
        [82][82][82]B.super.<init>();
        [76]()
      }
    }
  }
}
```

After fixing SI-4287
---------------------
```
[[syntax trees at end of                     typer]]
[21:82]package [29:36]org.com {
  [37:82]package [45:48]pkg {
    [50:57]abstract trait A extends [57:57][59]scala.AnyRef;
    [59:82]class B extends [76:82][82]scala.AnyRef {
      [77:81]<paramaccessor> private[this] val a: [77]org.com.pkg.A = _;
      <77:81>def <init>(<77:81>a: [80]<type: [80]A>): [76]org.com.pkg.B = <77:81>{
        <77:81><77:81><77:81>B.super.<init>();
        <77:81>()
      }
    }
  }
}
```