cmd/vet: unexported interface{} fields on %s are ok

For example, the following program is valid:

	type T struct {
		f interface{}
	}

	func main() {
		fmt.Printf("%s", T{"foo"}) // prints {foo}
	}

Since the field is of type interface{}, we might have any value in it.
For example, if we had T{3}, fmt would complain. However, not knowing
what the type under the interface is, we must be conservative.

However, as shown in #17798, we should issue an error if the field's
type is statically known to implement the error or fmt.Stringer
interfaces. In those cases, the user likely wanted the %s format to call
those methods. Keep the vet error in those cases.

While at it, add more field type test cases, such as custom error types,
and interfaces that extend the error interface.

Fixes #23563.

Change-Id: I063885955555917c59da000391b603f0d6dce432
Reviewed-on: https://go-review.googlesource.com/90516


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>