Do not confuse statement and argument names

Do not confuse statement and argument names

Our extension handling is still flaky, in that it assumes
that implicit extensions take an argument. As we progress
to make these special-cases handled, the confusion is being
flushed out.

This particular error was found when we made argument name
conditional on whether we actually are seeing an argument
being used in the model.

Change-Id: I4b8a0e3aa133343fc65dca38c1dd9e083113fb00
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 33a2fed5d2d2beee742825778be50c8eb133565b)