Make CC Mode load cl-lib rather than cl in Emacs 26.

Make CC Mode load cl-lib rather than cl in Emacs 26.

* lisp/progmodes/cc-cmds.el (c-declaration-limits): Remove unused local
variable.

* lisp/progmodes/cc-defs.el (c--mapcan-status): Remove.
(c--cl-library): New variable.
(Top level): Amend the form which requires library cl or cl-lib.
(c--mapcan, c--set-difference, c--intersection, c--macroexpand-all)
(c--delete-duplicate): Amend to use c--cl-library instead of
c--mapcan-status.

* lisp/progmodes/cc-engine.el (c-syntactic-skip-backward)
(c-back-over-compound-identifier): Remove unused local variables.

* lisp/progmodes/cc-fonts.el (c-font-lock-declarations): Remove an unused
local variable.

* lisp/progmodes/cc-langs.el (Top level): Amend to use c--cl-library instead
of c--mapcan-status.

* lisp/progmodes/cc-styles.el (Top level): Add a cc-bytecomp-defun to try to
silence a compiler warning.