DSLX ir_convert for parametric top with defaults

### What's hard to do? (limit 100 words)

Currently IR cannot be generated for a parametric function/proc. However, I expected this to be possible for a parametric top where _all parametrics have defaults_. This is not the case.

Related https://github.com/google/xls/issues/889, https://github.com/google/xls/issues/890.

### Current best alternative workaround (limit 100 words)

You need to write a wrapper (including replicating the type signature and passing all the args along) so that that the parametric gets all the default values.

### Your view of the "best case XLS enhancement" (limit 100 words)

The IR converter could attempt to default instantiate the parametric top.