By clearing out t.nod in copytype, we effectively lose the reference
from a Type back to its declaring OTYPE Node. This means later in
typenamesym when we add typenod(t) to signatlist, we end up creating a
new OTYPE Node. Moreover, this Node's position information will depend
on whatever context it happens be needed, and will be used for the
Type's position in the export data.

Updates #19391.

Change-Id: Ied93126449f75d7c5e3275cbdcc6fa657a8aa21d
Reviewed-on: https://go-review.googlesource.com/37870
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>