go/types: generate various source files from types2 files

go/types: generate various source files from types2 files

Starting point for more code sharing.

Change-Id: Ia6bc3ba54476a5202bfd5f89cef09bacb3f4f3b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/460761
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>