[dev.typeparams] cmd/compile/internal/types2: ensure that Named.check is...

[dev.typeparams] cmd/compile/internal/types2: ensure that Named.check is nilled out once it is expanded

This is a port of
- https://golang.org/cl/318849
- https://golang.org/cl/322974

For #45580.

Change-Id: Ie0700ed6c8d472305d5ba7ff97da1ae063152aa3
Reviewed-on: https://go-review.googlesource.com/c/go/+/323030


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>