go/types: parameterized functions must have a body

This is a port of CL 340911 to go/types. The new check differs slightly,
due to go/ast storing type parameters on the function type, rather than
declaration. The error was positioned on the function name for
consistency with types2 (and because that's a better position).

Change-Id: Icdfc76cd65fab215139180b710293a0d79709297
Reviewed-on: https://go-review.googlesource.com/c/go/+/342475


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