go/types: add a test that generated files match types2

go/types: add a test that generated files match types2

Adjust the go/types file generation to run in a test, so that we can
easily reuse the existing logic to verify that the current content of
go/types matches the expected result of generating from types2.

This test will enforce that we don't forget to regenerate go/types when
making changes to types2.

Change-Id: Iee14b1402065f7f0ecbcf28000e07a06c08fa42e
Reviewed-on: https://go-review.googlesource.com/c/go/+/462758
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>