bugfix: set assignment type for actual parameters correctly

[kroening]

A test requires a user. The compact-trace feature (#3458) is an opportunity for this.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 75dd6bb).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/92441920

[kroening]

Test will be added once #3458 is merged.

[kroening]

Test now added.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: d20b124).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/92944953