go/types: re-enable suppressed gcimporter tests

go/types: re-enable suppressed gcimporter tests

Port CL 577616 from types2 to go/types allows us
to re-enable these tests.

Fixes #66859.
Fixes #67436.
Updates #50259.
Updates #65294.

Change-Id: I573e51cf16546ac1c115beac5322b51dd998c881
Reviewed-on: https://go-review.googlesource.com/c/go/+/586236
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>