Currently if you declare a type overwriting a predeclared type
and export methods on it they will be exposed in godoc, even
though the type itself is not exported. This corrects that
by making all methods on these types hidden, since that's
the expected output.

Fixes #9860

Change-Id: I14037bdcef1b4bbefcf299a143bac8bf363718e0
Reviewed-on: https://go-review.googlesource.com/20610Reviewed-by: Russ Cox <rsc@golang.org>