reflect: name interface type that pins method info

I recently added TestUnexportedMethods which uses an interface type
to pin type information for an unexported method. But as written,
the interface type is not accessible to the reflect package.

You can imagine a future compiler optimization realizing that and
removing the type information for f. In fact, cl/20901 happens to
do that.

Change-Id: I1ddb67f50cb9b5737253b58f10545f3de652c29d
Reviewed-on: https://go-review.googlesource.com/21112Reviewed-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/all_test.go

@@ -2372,12 +2372,14 @@ type unexp struct{}
 func (*unexp) f() (int32, int8) { return 7, 7 }
 func (*unexp) g() (int64, int8) { return 8, 8 }
 
-func TestUnexportedMethods(t *testing.T) {
-	_ = (interface {
+type unexpI interface {
 	f() (int32, int8)
-	})(new(unexp))
+}
 
-	typ := TypeOf(new(unexp))
+var unexpi unexpI = new(unexp)
+
+func TestUnexportedMethods(t *testing.T) {
+	typ := TypeOf(unexpi)
 
 	if typ.Method(0).Type == nil {
 		t.Error("missing type for satisfied method 'f'")