DEFTRANSFORM placement of declarations is wrong when :Policy arg is used

[Douglas Katzman]

This bug concerns declarations in the transform's body code, not declarations that the transform's body generates.

* (sb-c::defknown grok (t) t (sb-c::foldable))
=> (GROK)
* (sb-c::deftransform grok ((x) * *) (declare (special ridiculous)) `(foo x))
=> GROK
* (sb-c::deftransform grok ((x) (vector) * :policy :fast) (declare (special ridiculous)) `(fast-foo x))

; in: SB-C:DEFTRANSFORM GROK
; (DECLARE (SPECIAL RIDICULOUS))
;
; caught WARNING:
; There is no function named DECLARE. References to DECLARE in some contexts
; (like starts of blocks) have special meaning, but here it would have to be a
; function, and that shouldn't be right.
[... rest of error text elided ...]

Expected result: Those should either both work (if it makes sense for DEFTRANSFORM to accept declarations at all), or both not work (if it doesn't).

The issue is simple - the expression (UNLESS ... ,@decls-body) is wrong. It stems from essentially the same code in CMUCL.

(defmacro deftransform (name (lambda-list &optional (arg-types '*) ...
    (let (...
          (decls-body `(,@decls ,@body)))
      (multiple-value-bind (parsed-form vars)
          (parse-deftransform lambda-list
                              (if policy
                                  `((unless (policy ,n-node ,policy)
                                      (give-up-ir1-transform))
                                    ,@decls-body)
                                  body)