Allow usage of AllowUnexported() and IgnoreFields()

[gauntface]

I have hit a few scenarios where I want to compare some unexported fields but not all and I can't use AllowUnexported and IgnoreFields together.

opts := []cmp.Option{
	cmp.AllowUnexported(Client{}),
	cmpopts.IgnoreFields(Client{}, "httpNewRequest"),
}
if diff := cmp.Diff(got, tt.want, opts...); diff != "" {
	t.Fatalf("Unexpected result; diff %v", diff)
}

Results in a panic: name must be exported

[dsnet]

It's been on my radar to have cmpopts.IgnoreFields work with unexported fields. If you want to try and enable this, feel free to send a PR.