go/types: generate typestring.go from types2 source
This CL reduces the amount of code that needs to be maintained
manually by about 500 LOC.
Change-Id: I643bea15203f7a916ef78b2738f125aa5b312eed
Reviewed-on: https://go-review.googlesource.com/c/go/+/565438 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>
Auto-Submit: Robert Griesemer <gri@google.com>