Don't assume built-in structs are mutable

Doing so adds spurious entries in the type environment for
the setters for these structs, even though the setters are
not defined.

typed-racket-lib/typed-racket/typecheck/tc-structs.rkt

@@ -330,7 +330,7 @@
   (define parent-tys (map fld-t (get-flds parent-type)))
 
   (define names (get-struct-names nm nm fld-names #f #f))
-  (define desc (struct-desc parent-tys tys null #t #f))
+  (define desc (struct-desc parent-tys tys null #f #f))
   (define sty (mk/inner-struct-type names desc parent-type))
 
   (register-sty! sty names desc)

typed-racket-test/fail/no-setters-for-built-in-structs.rkt

@@ -0,0 +1,8 @@
+#;
+(exn-pred #rx"missing type for top-level")
+#lang racket/load
+
+;; Test that built-in struct fields don't have types for setters
+
+(require typed/racket/base)
+set-date-second!