[dev.regabi] go/types: consistently report nil type as "untyped nil"
This is a port of CL 284052 to go/types. The port is not entirely
faithful, as untyped conversion has been refactored in go/types.
Additionally, a comment was added to reference issue #13061 in the
implicitType method.
For #13061
Change-Id: Iec17611f6432c988624023d1d74121ff34eb0c83
Reviewed-on: https://go-review.googlesource.com/c/go/+/289715
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>