comp-cstr.el: Fix a minor error and prepare for upcoming changes

comp-cstr.el: Fix a minor error and prepare for upcoming changes

* lisp/emacs-lisp/comp-cstr.el (comp--cl-class-hierarchy): Add `atom`
and `t` only to those types whose "allparents" is clearly not complete.
(comp--compute--pred-type-h): Store the cstr rather than the type
in the hash-table, as expected by `comp--pred-to-cstr`.

(cherry picked from commit 9830421e964cfb39077b69efd38d122e3bacf5d4)