go/types: generate mono.go from types2 source

go/types: generate mono.go from types2 source

This CL reduces the amount of code that needs to be maintained
manually by about 340 LOC.

Change-Id: If7b96c30e5a2f1ff28ebf4ca2f3ac3f73d6a8865
Reviewed-on: https://go-review.googlesource.com/c/go/+/565839
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>