* lisp/progmodes/cc-defs.el: Expose c-lanf-defconst's expressions to the

* lisp/progmodes/cc-defs.el: Expose c-lanf-defconst's expressions to the
byte-compiler.
(lookup-syntax-properties): Silence byte-compiler.
(c-lang-defconst): Quote the code with `lambda' rather than with
`quote'.
(c-lang-const): Avoid unneeded setq.
(c-lang-constants-under-evaluation): Add docstring.
(c-lang--novalue): New constant.
(c-find-assignment-for-mode): Use it instead of c-lang-constants.
(c-get-lang-constant): Same here.
Get the mode's value using `funcall' now that the code is quoted
with `lambda'.