[dev.typeparams] go/types: replace types2.Instantiate with Checker.Instantiate

[dev.typeparams] go/types: replace types2.Instantiate with Checker.Instantiate

This is a partial port of CL 333569 containing just changes to go/types.
Changes to the importer wil be made in a separate CL.

Change-Id: I9383e260b76402875ca6eb23c4478a6a3e8c1f0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/335071
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>