cmd/compile/internal/types2: merge Named type loading and expansion

Clean port of CL 349410 from go/types to types2 with 2 adjustments:
using syntax.Pos instead of token.Pos, and using TypeHash instead
of typeHash.

Fixes #47887.

Change-Id: Ifd8495e4187b5e30aaf80702768d82aad5e10cf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/349995


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