This CL reverts CL 76851 and takes a different approach to #21357.
The changes in encode.go and encode_test.go are reverts that
rolls back the changed behavior in CL 76851 where
embedded pointers to unexported struct types were
unilaterally ignored in both marshal and unmarshal.

Instead, these fields are handled as before with the exception that
it returns an error when Unmarshal is unable to set an unexported field.
The behavior of Marshal is now unchanged with regards to #21357.

This policy maintains the greatest degree of backwards compatibility
and avoids silently discarding data the user may have expected to be present.

Fixes #21357

Change-Id: I7dc753280c99f786ac51acf7e6c0246618c8b2b1
Reviewed-on: https://go-review.googlesource.com/82135
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>