cmd/compile/internal/types2: add test for imported constraints pre-1.18
But exclude the test when running unified build for now
(the unified builder's importers are not yet updated to
handle extended interfaces).
Also, fix respective error position.
Fixes #47967.
Change-Id: I4e3d829b5c12001c024b9eefcc27f97b10c1d1e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/363834
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>