go/types: address some TODOs (cleanup)

go/types: address some TODOs (cleanup)

This is a port of CL 345176 to go/types, though not all TODOs were
present in go/types.

A TODO that still needs to be resolved was added back to types2.

Change-Id: Icf79483c92d0bc1248de772c7044620f0f0a5c58
Reviewed-on: https://go-review.googlesource.com/c/go/+/346550
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>