go/types: : generate assignments.go from types2 source

go/types: : generate assignments.go from types2 source

This CL reduces the amount of code that needs to be maintained
manually by about 600 LOC.

Change-Id: I7107c8050075281bf6840a9f5234d70e09734ce6
Reviewed-on: https://go-review.googlesource.com/c/go/+/565836
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>