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

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

Change-Id: Ibecf2b5bc982f6bf92310267b9f06b588b7148a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/291169


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