Inline important-return-value declarations in cl-lib.el

These declarations are now properly added to 'cl-lib.el' itself, or to
'cl-loaddefs.el'.  This means that they will now correctly show up
immediately when loading 'cl-lib.el', instead of only after 'cl-macs.el'
is pulled in by an autoload.  C.f. Bug#76247.

I did not considered worth reproducing everywhere the list saying which
functions among the below belong to these two categories:
1. Functions that are side-effect-free except for the behavior of
   functions passed as argument.
2. Functions that mutate and return a list.

AFAIU, this is not actionable with our current byte-compiler, i.e. we
can't add any extra declarations based on it.  However, if the list
should be needed at some point, for example due to improvements in the
compiler, this commit will be where to find it.  In the worst case, and
with more work, it's also deducible from the source code itself.

* lisp/emacs-lisp/cl-macs.el: Move important-return-value declarations
from here...
* lisp/emacs-lisp/cl-extra.el (cl-map, cl-maplist, cl-mapcan)
(cl-mapcon, cl-some, cl-every, cl-notany, cl-notevery, cl-nreconc):
* lisp/emacs-lisp/cl-lib.el (cl-mapcar, cl-adjoin, cl-subst):
* lisp/emacs-lisp/cl-seq.el (cl-reduce, cl-remove, cl-remove-if)
(cl-remove-if-not, cl-delete, cl-delete-if, cl-delete-if-not)
(cl-remove-duplicates, cl-delete-duplicates, cl-substitute)
(cl-substitute-if, cl-substitute-if-not, cl-nsubstitute)
(cl-nsubstitute-if, cl-nsubstitute-if-not, cl-find, cl-find-if)
(cl-find-if-not, cl-position, cl-position-if, cl-position-if-not)
(cl-count, cl-count-if, cl-count-if-not, cl-mismatch, cl-search)
(cl-sort, cl-stable-sort, cl-merge, cl-member, cl-member-if)
(cl-member-if-not, cl-assoc, cl-assoc-if, cl-assoc-if-not, cl-rassoc)
(cl-rassoc-if, cl-rassoc-if-not, cl-union, cl-nunion, cl-intersection)
(cl-nintersection, cl-set-difference, cl-nset-difference)
(cl-set-exclusive-or, cl-nset-exclusive-or, cl-subsetp, cl-subst-if)
(cl-subst-if-not, cl-nsubst, cl-nsubst-if, cl-nsubst-if-not, cl-sublis)
(cl-nsublis, cl-tree-equal): ...to have them inline here.

lisp/emacs-lisp/cl-lib.el

@@ -369,6 +369,7 @@ and mapping stops as soon as the shortest list runs out.  With just one
 SEQ, this is like `mapcar'.  With several, it is like the Common Lisp
 `mapcar' function extended to arbitrary sequence types.
 \n(fn FUNCTION SEQ...)"
+  (declare (important-return-value t))
   (if cl-rest
       (if (or (cdr cl-rest) (nlistp cl-x) (nlistp (car cl-rest)))
 	  (cl--mapcar-many cl-func (cons cl-x cl-rest) 'accumulate)
@@ -506,7 +507,8 @@ The elements of LIST are not copied, just the list structure itself."
 Otherwise, return LIST unmodified.
 \nKeywords supported:  :test :test-not :key
 \n(fn ITEM LIST [KEYWORD VALUE]...)"
-  (declare (compiler-macro cl--compiler-macro-adjoin))
+  (declare (important-return-value t)
+           (compiler-macro cl--compiler-macro-adjoin))
   (cond ((or (equal cl-keys '(:test eq))
 	     (and (null cl-keys) (not (numberp cl-item))))
 	 (if (memq cl-item cl-list) cl-list (cons cl-item cl-list)))
@@ -519,6 +521,7 @@ Otherwise, return LIST unmodified.
 Return a copy of TREE with all elements `eql' to OLD replaced by NEW.
 \nKeywords supported:  :test :test-not :key
 \n(fn NEW OLD TREE [KEYWORD VALUE]...)"
+  (declare (important-return-value t))
   (if (or cl-keys (and (numberp cl-old) (not (integerp cl-old))))
       (apply 'cl-sublis (list (cons cl-old cl-new)) cl-tree cl-keys)
     (cl--do-subst cl-new cl-old cl-tree)))