Fix rx `regexp` form with deprecated syntax

Fix rx `regexp` form with deprecated syntax

The argument of the rx `regexp` form is assumed to evaluate to a valid
regexp, but certain kinds of deprecated but still accepted usage were
not handled correctly, such as unescaped literal (special) characters:
 (rx "a" (regexp "*")) => "a*" which is wrong.
Handle these cases; there is no extra trouble.

* lisp/emacs-lisp/rx.el (rx--translate-regexp): Force bracketing
of single special characters.
* test/lisp/emacs-lisp/rx-tests.el (rx-regexp): Add test case.