spec: adjust assignability rules for type parameters
spec: adjust assignability rules for type parameters

Change-Id: I3c4d8bdb5e92ee7fdca9593fb043f94f467755e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/365434
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go