go/types: handle recursive type parameter constraints

This is a port of CL 348090 to go/types. Notably, unlike in types2,
declareTypeParams was previously setting the default constraint to the
empty interface, not nil, because this was missed in CL 335034 (no
changes were made to declareTypeParams). This CL fixes this discrepancy.

Change-Id: I0fa54a660ba14c6cbefa81a27ab7eb193df3be20
Reviewed-on: https://go-review.googlesource.com/c/go/+/348690


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>