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

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.

(cherry picked from commit f05ce9e6bbe7202060580c122e033f5e4ac98e90)