Fix misfontification of C++ member initialization list after "throw"

Fix misfontification of C++ member initialization list after "throw"

* lisp/progmodes/cc-engine.el (c-forward-type): Stop recognizing a "type"
starting with "throw", by using c-opt-type-modifier-prefix-key.

* lisp/progmodes/cc-langs.el (c-type-modifier-prefix-kwds): New lang const
which, in C++, doesn't contain "throw", otherwise like c-type-modifier-kwds.
(c-opt-type-modifier-prefix-key): New lang const and var, a regexp matching
any keyword in the previous lang const.