[bugfix] Fix naming module with Unit parameter

Fix a bug in naming where a Module that took a Unit parameter could cause
naming to crash.  This is, admittedly, a very weird pattern, but can occur
if you have a type-parametric module where certain type parameterizations
can be Unit.

Fix #3245.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>