[dev.typeparams] cmd/compile/internal/types: review of type.go

The changes between (equivalent, and reviewed) go/types/type.go
and type.go can be seen by comparing patchset 1 and 3. The actual
change is just removing the "// UNREVIEWED" marker and some
comment adjustments.

Change-Id: Ied0e2f942bc96a9fcae0466761cfaa60a87668db
Reviewed-on: https://go-review.googlesource.com/c/go/+/293471


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