cmd/compile: add two error position tests for the typechecker

Follow CL 41477 and add two more line position tests for yyerror calls
in the typechecker which are currently not tested.

Update #19683

Change-Id: Iacd865195a3bfba87d8c22655382af267aba47a9
Reviewed-on: https://go-review.googlesource.com/70251Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

test/method2.go

@@ -35,3 +35,7 @@ var pv = &v
 
 var _ = pv.val() // ERROR "pv.val undefined"
 var _ = pv.val   // ERROR "pv.val undefined"
+
+func (t *T) g() int { return t.a }
+
+var _ = (T).g() // ERROR "needs pointer receiver|undefined"

test/method6.go

+// 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.
+
+// Verify that pointer method calls are caught during typechecking.
+// Reproducer extracted and adapted from method.go
+
+package foo
+
+type A struct {
+	B
+}
+type B int
+
+func (*B) g() {}
+
+var _ = func() {
+	var a A
+	A(a).g() // ERROR "cannot call pointer method on|cannot take the address of"
+}