cmd/compile/internal/types2: produce empty type set for invalid ~T
If ~T is not permitted because the underlying type of T is not the
same as T, there is no type that satisfies ~T. Besides reporting an
error, also ensure that the corresponding type set is empty.
For #49739.
Change-Id: I127f75f170902e7989f7fe7b352dabda9f72e2a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/366278
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>