go/types: accept constraint literals with elided interfaces

This is a port of CL 353139 to go/types, adjusted for error reporting
and for the different representation of field lists in go/ast. A TODO is
added to verify if types2 produces redundant error messages for type
parameters sharing a bound.

For #48424

Change-Id: I3549942be0328de616d1d87d0ba621311fc53576
Reviewed-on: https://go-review.googlesource.com/c/go/+/354989


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>