[dev.typeparams] cmd/compile: fix handling of Nname field in (*subster).tstruct.

We want to keep the Nname references for external function references in
tstruct (not remove them, as is currently happening). We only change the
Nname reference (translate it) when it appears in subst.vars[].

New export/import test sliceimp.go which includes some of these external
function references.

Change-Id: Ie3d73bd989a16082f0cebfb566e0a7faeda55e60
Reviewed-on: https://go-review.googlesource.com/c/go/+/321735


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>