go/types: generate conversions.go from types2 source

go/types: generate conversions.go from types2 source

This CL reduces the amount of code that needs to be maintained
manually by about 300 LOC.

Change-Id: I749e47668e90e77e99109005fbe19045d4a5ad29
Reviewed-on: https://go-review.googlesource.com/c/go/+/565437
Reviewed-by: Robert Findley <rfindley@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 Griesemer <gri@google.com>