Fontify C, C++, Objective C identifiers containing $

Fontify C, C++, Objective C identifiers containing $

Optionally, fontify them with font-lock-warning-face.
This fixes bug#64204.

* lisp/progmodes/cc-fonts.el (c-font-lock-ids-with-dollar):
New function.
(c-simple-decl-matchers, c-complex-decl-matchers): invoke
c-font-lock-ids-with-dollar for pertinent languages.

* lisp/progmodes/cc-langs.el (c-symbol-start): Add `$' to the
character list.
(c-dollar-in-ids): New lang const.
(c-symbol-key): For the Pike value, use the AWK value rather
than the C value as the basis, as the latter is no longer
suitable.

* lisp/progmodes/cc-vars.el (c-warn-ids-with-dollar): New
customizable option.

* doc/misc/cc-mode.texi ("Miscellaneous Font Locking"): Add a
section on the new optional fontification of identifiers with
'font-lock-warning-face'.