go/types: slice exprs to accept type sets with single underlying types

go/types: slice exprs to accept type sets with single underlying types

This is a port of CL 357779 to go/types. A test error message was
repositioned on the sliced operand.

Change-Id: Ie775c128f70d9cb08a2eba54b8bc082134ec3200
Reviewed-on: https://go-review.googlesource.com/c/go/+/359876
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>