reflect: fix strings of SliceOf-created types

The new type was inheriting the tflagExtraStar from its prototype.

Fixes #15467

Change-Id: Ic22c2a55cee7580cb59228d52b97e1c0a1e60220
Reviewed-on: https://go-review.googlesource.com/22501Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/reflect/all_test.go

@@ -3902,6 +3902,9 @@ func TestSliceOf(t *testing.T) {
 	// check construction and use of type not in binary
 	type T int
 	st := SliceOf(TypeOf(T(1)))
+	if got, want := st.String(), "[]reflect_test.T"; got != want {
+		t.Errorf("SliceOf(T(1)).String()=%q, want %q", got, want)
+	}
 	v := MakeSlice(st, 10, 10)
 	runtime.GC()
 	for i := 0; i < v.Len(); i++ {

src/reflect/type.go

@@ -2251,6 +2251,7 @@ func SliceOf(t Type) Type {
 	prototype := *(**sliceType)(unsafe.Pointer(&islice))
 	slice := new(sliceType)
 	*slice = *prototype
+	slice.tflag = 0
 	slice.str = resolveReflectName(newName(s, "", "", false))
 	slice.hash = fnv1(typ.hash, '[')
 	slice.elem = typ