DSL macro type is not subtypable

[countvajhula]

@samph reports that this type is not subtypable for a custom macro-defining form that he was interested in writing in connection with the Uke library.

(begin-for-syntax
  (struct my-qi qi-macro ()))
;; struct: parent struct type not defined;
;;  identifier does not name struct type information in: qi-macro

Formerly qi-macro was a type defined in the Qi codebase, and we later transitioned to using Syntax Spec directly. I verified that the former definition is subtypable using the above example code from Sam, but the latter (current version) isn't.

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