Put Instance types in the seen list for subtyping

Since we resolve under Instance types for subtyping we
need to put them in the current-seen list too. Fixes
an infinite loop bug in subtyping.

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/types/current-seen.rkt

@@ -17,6 +17,7 @@
 (define (remember s t A) 
   (if (or (Mu? s) (Mu? t)
           (Name? s) (Name? t)
+          (Instance? s) (Instance? t)
           (Struct? s) (Struct? t) 
           (App? s) (App? t))
       (cons (seen-before s t) A)

pkgs/typed-racket-pkgs/typed-racket-test/tests/typed-racket/unit-tests/class-tests.rkt

@@ -1651,4 +1651,20 @@
            (: d% D)
            (define d% (class object% (super-new) (define/public (m) (void))))
            (send (new d%) m))
-         Univ]))
+         Univ]
+   ;; Test for an infinite loop bug during subtyping for classes
+   [tc-e (let ()
+           (define-type-alias A% (Class [get-this (-> (Instance A%))]))
+           (define-type-alias B% (Class [get-this (-> (Instance B%))]))
+           (: a% A%)
+           (define a%
+             (class object%
+               (super-new)
+               (define/public (get-this) this)))
+           (: b% B%)
+           (define b%
+             (class a%
+               (super-new)
+               (define/override (get-this) this)))
+           (void))
+         -Void]))