go/types, types2: move functions for untyped constants into const.go

go/types, types2: move functions for untyped constants into const.go

No changes to the moved functions.
Generate const.go for go/types.

Change-Id: I5ac412cecd9f618676a01138aed36428bbce3311
Reviewed-on: https://go-review.googlesource.com/c/go/+/493715
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>