[dev.typeparams] cmd/compile/internal/types2: replace Named, TypeParam methods with functions

This removes two more converter methods in favor of functions.
This further reduces the API surface of types2.Type and matches
the approach taken in go/types.

Change-Id: I3cdd54c5e0d1e7664a69f3697fc081a66315b969
Reviewed-on: https://go-review.googlesource.com/c/go/+/293292


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>