The structure of the code meant that an embedded field was never
checked for export status. We need to check the name of the type,
which is either of type T or type *T, and T might be unexported.

Fixes #14356.

Change-Id: I56f468e9b8ae67e9ed7509ed0b91d860507baed2
Reviewed-on: https://go-review.googlesource.com/19701Reviewed-by: Robert Griesemer <gri@golang.org>