go/types: refactor the Context type map to accept arbitrary types
In preparation for storing *Signature types in Context, refactor the
type map to not depend on the *Named type API.
Change-Id: I0439d43aa4cc3a60a78f409a773a343a4fffd0fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/362799
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>