fix `syntax/boundmap', too

Related to PR 13911

pkgs/racket-pkgs/racket-test/tests/racket/boundmap-test.rktl

@@ -143,4 +143,11 @@
                  (set! l (cons y l))))
               l)))))
 
+(let ()
+  (define-syntax name 'dummy)
+  (define-syntax alias (make-rename-transformer #'name))
+  (define table (make-free-identifier-mapping))
+  (free-identifier-mapping-put! table #'alias 0)
+  (test 0 free-identifier-mapping-get table #'name))
+
 (report-errs)

racket/lib/collects/syntax/private/boundmap.rkt

@@ -107,10 +107,7 @@
   (define-struct module-identifier-mapping (ht))
 
   (define (module-identifier->symbol id) 
-    (let ([binding (identifier-binding id)])
-      (if (pair? binding)
-          (cadr binding)
-          (syntax-e id))))
+    (identifier-binding-symbol id))
   
   (make-mapping-code
    module-identifier->symbol