go/types: factor out slice elem computation for copy built-in
This is a port of CL 357413 to go/types. Some test constraints are also
updated to remove 'interface', to coincide with the corresponding test
data file in types2.
Change-Id: I5248190501c2e4381eb7625f8d4fb269301d6e16
Reviewed-on: https://go-review.googlesource.com/c/go/+/359138
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>