CC Mode: Handle C++20's if consteval

CC Mode: Handle C++20's if consteval

* lisp/progmodes/cc-engine.el (c-after-conditional): Handle the
new keyword in place of a paren sexp after `if'.

* lisp/progmodes/cc-langs.el (c-negation-op-re)
(c-paren-clause-kwds, c-paren-clause-key)
(c-block-stmt-with-kwds, c-block-stmt-with-key): New
lang-consts/vars.

* if-11.cc, if-11.res: New test files.

(cherry picked from commit ed85132740b39c147647be1831abb64a3f514d57)