[dev.typeparams] go/types: implement close(ch) where ch is of type parameter type

[dev.typeparams] go/types: implement close(ch) where ch is of type parameter type

This is a port of CL 333713 to go/types.

Change-Id: I517f52592f65cc76e11a12d9148b20c12d9e3e81
Reviewed-on: https://go-review.googlesource.com/c/go/+/335077
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>