byte-compile-preprocess+cconv seem to mutate self evaluating forms in expanded macros

[Vibhav Pant]

While working on scratch/comp-static-data, which adds support for
native compiling self evaluating forms in Lisp code as static consts in
the eln (with immutability enforced additionally by `CHECK_IMPURE`), I
noticed that compiling minor-mode definitions would result in a
`pure_write_error`. This arises as cconv-closure-convert, called from
`byte-compile-preprocess` would call `setcar` on a self evaluating
interactive form as part of the minor-mode's function body.

A hacky way to reproduce this would be to stuff the following snippet
into loadup.el (purecopy ensures modifying the list triggers an error):

```
(load "emacs-lisp/bytecomp")
(setq sample-interactive-spec
      (purecopy '(interactive
                  (list (if current-prefix-arg
                            (prefix-numeric-value 
                             current-prefix-arg)
                          'toggle)))))

(defmacro define-purecopied-func ()
  `(defun foo-bar (arg)
     ,sample-interactive-spec))

(let ((byte-compile-debug t))
  (byte-compile '(define-purecopied-func)))
```
(Alternatively, just building scratch/comp-static-data will also
reproduce these errors)

Because modifying self-evaluating forms is technically undefined
behaviour as per the Elisp reference manual
(https://www.gnu.org/software/emacs/manual/html_node/elisp/Mutability.html),
shouldn't we be use 'copy-tree' after macro expansion in these cases?


-- 
Vibhav Pant
vibhavp@gmail.com
GPG: 7ED1 D48C 513C A024 BE3A  785F E3FB 28CB 6AB5 9598

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