Fix some side-effecting uses of make-text-button
For discussion, see the following thread: https://lists.gnu.org/archive/html/emacs-devel/2020-06/msg00117.html * lisp/apropos.el (apropos-library-button): * lisp/help-fns.el (help-fns--first-release): Return result of make-text-button instead of relying on its side effects. * lisp/ibuf-ext.el (ibuffer-old-saved-filters-warning): Avoid modifying an immutable string.