[dev.typeparams] cmd/compile/internal/types2: support local defined types

This CL changes types2's instance hashing logic to include position
information for function-scope defined types as disambiguation. This
isn't ideal, but it worked for getting nested.go passing.

Updates #46592.

Change-Id: Id83ba0001f44af69b81260306cc8b05e44fc4f09
Reviewed-on: https://go-review.googlesource.com/c/go/+/327170


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