reflect: clear tflag for StructOf type

Fixes #15923

Change-Id: I3e56564365086ceb0bfc15db61db6fb446ab7448
Reviewed-on: https://go-review.googlesource.com/23760Reviewed-by: Sebastien Binet <seb.binet@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/all_test.go

@@ -3934,6 +3934,10 @@ func TestStructOf(t *testing.T) {
 	if s != want {
 		t.Errorf("constructed struct = %s, want %s", s, want)
 	}
+	const stStr = `struct { S string "s"; X uint8 "x"; Y uint64; Z [3]uint16 }`
+	if got, want := st.String(), stStr; got != want {
+		t.Errorf("StructOf(fields).String()=%q, want %q", got, want)
+	}
 
 	// check the size, alignment and field offsets
 	stt := TypeOf(struct {

src/reflect/type.go

@@ -2640,6 +2640,7 @@ func StructOf(fields []StructField) Type {
 	}
 
 	typ.str = resolveReflectName(newName(str, "", "", false))
+	typ.tflag = 0
 	typ.hash = hash
 	typ.size = size
 	typ.align = typalign