cmd/compile/internal/types2: review of lookup.go

The changes between (equivalent, and reviewed) go/types/lookup.go
and lookup.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker.

Note: The function ptrRecv in types2/lookup.go is found in
      methodset.go in go/types (methodset.go doesn't exist
      in types2).

Change-Id: I48cfd3df0947becb4c3b5e55b89263917bcfbf16
Reviewed-on: https://go-review.googlesource.com/c/go/+/304129


Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>