go/types, types2: factor out type parameter access into genericType
Also, remove types2.Signature.SetTypeParams as it is not used
and does not exist in go/types.
Change-Id: I16c3ae988988d3735907e9c6c56e8626497ea405
Reviewed-on: https://go-review.googlesource.com/c/go/+/581817 Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>