go/types: fix range over exprs of type parameter type

This is a port of CL 339897 to go/types. In addition, an error message
that was adjusted in CL 274974 is ported to go/types (CL 274974 was
only considered necessary for compiler compatibility).

Change-Id: Idfe44d759c925f9fed353a2d1898d3d4d8d85452
Reviewed-on: https://go-review.googlesource.com/c/go/+/342433


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>