go/types: add Alias.Rhs

go/types: add Alias.Rhs

This method returns the type on the right-hand side of an
alias declaration such as type L = R.

Fixes #66559

Change-Id: I396f2d999680ad251f47cdde20856ae20fc1c40a
Reviewed-on: https://go-review.googlesource.com/c/go/+/581615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>