go/types, types2: wrap Named.fromRHS into Named.rhs

go/types, types2: wrap Named.fromRHS into Named.rhs

In debug mode, the Named.rhs method asserts that Named is in a state
with Named.fromRHS populated.

This caught a missing call to Named.unpack in validtype, which has been
added.

Change-Id: Id3f95f78f03d98a6efe87af6ac24f2ac2e285f96
Reviewed-on: https://go-review.googlesource.com/c/go/+/714242
Auto-Submit: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>