SI-8117 Fix bug when mixing well-positioned named and positional args

The method `missingParams` which returns undefined parameters of
a given invocation expression still assumed that named arguments can
only appear after positional ones.
The default argName function is kind of weird.  It has to take an arbitrary T, i.e., Any, so it's odd to supply a default that is not just a backup for the most general, useless case.  (This is existing code.)