Revert type key change from 0a6537a6cb

Sets are still treated as hash sets. Eventually
when we have a type for generic sets, it should have
a #f type key.

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/rep/type-rep.rkt

@@ -182,7 +182,7 @@
 
 ;; elem is a Type
 (def-type Set ([elem Type/c])
-  [#:key #f])
+  [#:key 'set])
 
 ;; result is a Type
 (def-type Evt ([result Type/c])

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

@@ -1817,6 +1817,14 @@
           (for: ([k : Symbol (in-set (set 'x 'y 'z))]) (displayln k))
           (void))
         -Void]
+
+       ;; PR 14139
+       [tc-e
+        (let ()
+          (: f : (U (Setof Integer) Integer) → (Setof Integer))
+          (define (f s) (if (set? s) s (set)))
+          (void))
+        -Void]
         )
   (test-suite
    "tc-literal tests"