cmd/compile: preserve Type.nod in copytype

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>

src/cmd/compile/internal/gc/bimport.go

@@ -404,12 +404,10 @@ func (p *importer) newtyp(etype EType) *Type {
 // importtype declares that pt, an imported named type, has underlying type t.
 func (p *importer) importtype(pt, t *Type) {
 	if pt.Etype == TFORW {
-		n := pt.nod
 		copytype(pt.nod, t)
-		pt.nod = n // unzero nod
 		pt.Sym.Importdef = importpkg
 		pt.Sym.Lastlineno = lineno
-		declare(n, PEXTERN)
+		declare(pt.nod, PEXTERN)
 		checkwidth(pt)
 	} else {
 		// pt.Orig and t must be identical.

src/cmd/compile/internal/gc/typecheck.go

@@ -3566,7 +3566,7 @@ func copytype(n *Node, t *Type) {
 	}
 	t.methods = Fields{}
 	t.allMethods = Fields{}
-	t.nod = nil
+	t.nod = n
 	t.SetDeferwidth(false)
 	t.ptrTo = ptrTo
 	t.sliceOf = sliceOf