go/types: factor out type parameter renaming (cleanup)

go/types: factor out type parameter renaming (cleanup)

Follow-up on CL 461687 which missed the go/types change.

(As an aside, we cannot yet generate this change because
go/types uses a positioner and types2 just uses a syntax.Pos.)

Change-Id: I28113a2efdc3ddd30cb9a80d2cb2c802ff035ec2
Reviewed-on: https://go-review.googlesource.com/c/go/+/461688
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>