Fix fontification bugs with constructors and const.

Fix fontification bugs with constructors and const.

* progmodes/cc-engine.el (c-forward-decl-or-cast-1): (Just after
CASE 2) Remove the check for the absence of a suffix construct
after a function declaration with only types (no identifiers) in
the parentheses.  Also, accept a function declaration with just a
type inside the parentheses, if this type can be positively
recognised as such, or if a prefix keyword like "explicit" nails
down the construct as a declaration.