go/types, types2: use nil to represent incomplete explicit aliases

Using Invalid to represent an incomplete alias is problematic since
it implies that an error has been reported somewhere. This causes
confusion for observers of invalid aliases trying not to emit
follow-on errors.

This change uses nil instead to represent an incomplete alias. This
has a mild benefit of making alias memoization more convenient. We
additionally can now memoize Invalid aliases.

This necessitates a minor change to our cycle error reporting for
aliases. Care is taken to separate logic according to gotypesalias.
Otherwise, a cycle as simple as "type T = T" panics.

A test is also added which uses go/types to inspect for Invalid
types. Currently, the problematic Invalid does not cause an error
in type checking, but rather a panic in noding. Thus, we cannot use
the familiar test facilities relying on error reporting.

Fixes #74181

Change-Id: Iea5ebce567a2805f5647de0fb7ded4a96f6c5f8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/683796
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>