Fix TermRef prefixes not having their type healed

[jchyb]

Fixes #19767

In the minimization from the issue, after the splicing phase we ended up with a quote with illegal k1 types, which should have been healed during the splicing phase:

'<k1$given2>{
  new ICons[(ICons[k1]#key : k1)](
  //...

To fix that, we now map over and heal the prefix of the TermRef as well, so we end up with:

'<k1$given2>{
  new ICons[(ICons[k1$given2]#key : k1$given2)](
  //...