reflect: permit unexported fields in StructOf

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.10.2 linux/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

package main

import "fmt"
import "reflect"

func main() {
	type T = struct {
		x int
	}
	fmt.Println(reflect.TypeOf(T{})) // struct { x int }
	
	var n int
	tn := reflect.TypeOf(n)
	// panic: reflect.StructOf: field "x" is unexported but missing PkgPath
	tt := reflect.StructOf([]reflect.StructField{
		{Name: "x", Type: tn, Anonymous: false},
	})
	fmt.Println(tt)
}

What did you expect to see?

not panic.

What did you see instead?

panic.

If it really should panic, it should be documented.