cmd/compile/internal/types2: better error messages for empty type sets

- change _TypeSet.hasTerms() to report if a type set has actual types
  (excluding a "universe" term)
- handle empty type set type arguments correctly
- bring comments up-to-date in Checker.satisfies

Change-Id: I87f9a1096ebb21a1b08c87a9b59f400f3bc2f040
Reviewed-on: https://go-review.googlesource.com/c/go/+/358175


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>