Replace lisp-mode-symbol-regexp with (rx lisp-mode-symbol)

Replace lisp-mode-symbol-regexp with (rx lisp-mode-symbol)

This is shorter, simplifies use inside rx expressions, and removes
need for eval-when-compile elsewhere (for later exploitation).

* lisp/emacs-lisp/lisp-mode.el (lisp-mode-symbol): New rx-define.
(lisp-mode-symbol-regexp): Redefine using lisp-mode-symbol.
(lisp-imenu-generic-expression, lisp--el-match-keyword)
(lisp-fdefs, lisp-string-in-doc-position-p):
* lisp/emacs-lisp/checkdoc.el (checkdoc--error-bad-format-p):
* lisp/emacs-lisp/shorthands.el (shorthands-font-lock-shorthands):
Use lisp-mode-symbol instead of lisp-mode-symbol-regexp.