Missing case types.TUNSAFEPTR in parameterizedBy().
Also realized there was the same missing case in the type substituter
(*Tsubster).Typ().
Fixes#48103
Change-Id: If71f1a6ef80932f0e2120d4c18b39a30189fd8fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/346669
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>