cmd/compile: report correct position in redeclaration errors

When restoring syms, we must also restore the original Lastlineno.
Bug introduced with https://golang.org/cl/41390/.

Fixes #20415.

Change-Id: Ie81d36279d717e330951b52f42dcee4b0025b9f0
Reviewed-on: https://go-review.googlesource.com/43811
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/types/scope.go

@@ -22,6 +22,7 @@ func dcopy(a, b *Sym) {
 	a.Name = b.Name
 	a.Def = b.Def
 	a.Block = b.Block
+	a.Lastlineno = b.Lastlineno
 }
 
 func push() *Sym {

src/cmd/compile/internal/types/sym.go

@@ -20,13 +20,13 @@ import (
 type Sym struct {
 	Importdef *Pkg   // where imported definition was found
 	Linkname  string // link name
-	Lastlineno src.XPos // last declaration for diagnostic
 
 	// saved and restored by dcopy
 	Pkg        *Pkg
 	Name       string   // object name
 	Def        *Node    // definition: ONAME OTYPE OPACK or OLITERAL
 	Block      int32    // blocknumber to catch redeclaration
+	Lastlineno src.XPos // last declaration for diagnostic
 
 	flags   bitset8
 	Label   *Node // corresponding label (ephemeral)

test/fixedbugs/issue20415.go

@@ -0,0 +1,33 @@
+// errorcheck
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure redeclaration errors report correct position.
+
+package p
+
+// 1
+var f byte
+
+var f interface{} // ERROR "previous declaration at issue20415.go:12"
+
+func _(f int) {
+}
+
+// 2
+var g byte
+
+func _(g int) {
+}
+
+var g interface{} // ERROR "previous declaration at issue20415.go:20"
+
+// 3
+func _(h int) {
+}
+
+var h byte
+
+var h interface{} // ERROR "previous declaration at issue20415.go:31"