[dev.typeparams] go/types: use InstantiateLazy to create instance types (cleanup)
This is a port of CL 333669 to go/types, adjusted for the position and
IndexExpr APIs, and excluding the noder changes.
Change-Id: I3ac4bbf271947c3cf80ab04c462a91657316f4fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/335073
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>