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

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

Change-Id: Ic40d1a9b2f1465c5335bd69e9a0b265ab694c3ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/291170


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