gh-94808: Reorganize _make_posargs and mark unused code (GH-119227)

* Reorganize four-way if-elsif-elsif-elsif as nested if-elses
* Mark unused branch in _make_posargs

`names_with_default` is never `NULL`, even if there are no names with
defaults.  In that case it points to a structure with `size` zero.

Rather than eliminating the branch, we leave it behind with an `assert(0)`
in case a future change to the grammar exercises the branch.