go/types: generate typeset.go from types2 source

go/types: generate typeset.go from types2 source

To simplify the translation, use extra atPos calls where needed
in the respective types2 source.

This CL reduces the amount of code that needs to be maintained
manually by about 420 LOC.

Change-Id: I839844a6e85ccb1111d76c43de23127d8f9fbbce
Reviewed-on: https://go-review.googlesource.com/c/go/+/567776
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>