add #line directive to generated C code to support source debugging

sexc.scm

@@ -1,6 +1,7 @@
 (declare (unit sexc)
          (uses fmt-c-writer
                sex-reader
+               utils
                semen))
 
 (include "utils.macros.scm")
@@ -154,6 +155,7 @@
          (return #f))
        (load-persistent-module-paths)
 
+       (sex-fmt-current-file (to-absolute-pathname input))
        (let* ((raw-forms (append prelude (read-raw-forms input)))
               (sex-forms (semantic-process-forms raw-forms input)))
          (if (or (get-arg args 'macro-expand #f)