Handle C++17's constexpr keyword in if statements

Handle C++17's constexpr keyword in if statements

* lisp/progmodes/cc-engine.el (c-after-conditional): Test for matches to
c-block-stmt-hangon-key.

* lisp/progmodes/cc-langs.el (c-block-stmt-hangon-kwds): New lang const.
(c-block-stmt-hangon-key): New lang const/var matching any element of the
above.