SI-7915 Corrected range positions created during default args expansion

The tree created during expansion of default arguments contained trees with
the wrong type of positions. In particular:

* The synthetic $qual should have a transparent position.
* The created Select should always have a range position, because `selected`
  (i.e., the called method) is always visible in the source.
* The Block that contains the expanded default arguments is transparent.

The only tricky part of the fix is the position assigned to the new Select tree,
which needs to respect the range position invariants. In the specific case, we
ought to make sure that range positions don't overlap. Therefore, the position
assigned to the new Select tree is computed by intersecting the original Select
position (i.e., `baseFun`'s position) and the original qualifier's position
(i.e., `qual`'s position).