[dev.typeparams] go/types: backport lazy loading changes from CL 336252

When CL 336252 was created (itself a port of CL 335929), types2
tests revealed that lazy expansion of instances was not behaving
correctly with respect to lazy loading of Named types.

This CL ports the fixes from CL 336252 back to go/types.

Change-Id: Iffc6c84a708449633153b800dfb98ff57402893c
Reviewed-on: https://go-review.googlesource.com/c/go/+/338369


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>