Marshal must process unexported embedded fields of struct type,
looking for exported fields in those structs. However, it must
not process unexported embedded fields of non-struct type.

For example, consider:

    type t1 struct {
        X int
    }
    type t2 int
    type T struct {
        t1
        t2
    }

When considering T, Marshal must process t1 to find t1.X.
Marshal must not process t2, but it was. Fix that.

Fixes #18009

Change-Id: I62ba0b65ba30fd927990e101a26405a9998787a3
Reviewed-on: https://go-review.googlesource.com/33773
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>