go/doc: propagate types from unexported constants
When constants were declared using unexported constants,
the type information was lost when those constants were filtered out.
This CL propagates the type information of unexported constants
so that it is available for display.
This is a follow-up to CL
144110044, which fixed this problem
specifically for _ constants.
Updates #5397.
Change-Id: I3f0c767a4007d88169a5634ab2870deea4e6a740
Reviewed-on: https://go-review.googlesource.com/2091
Reviewed-by: Robert Griesemer <gri@golang.org>