Fix irregularities with CC Mode fontification, particularly with "known types"

Fix irregularities with CC Mode fontification, particularly with "known types"

* lisp/progmodes/cc-fonts.el (c-font-lock-declarators): Introduce a new
optional parameter, template-class.  In "class <X = Y>", fontify "Y" as a
type.
(c-font-lock-single-decl): New variable template-class, set to non-nil when we
have a construct like the above.  Pass this as argument to
c-font-lock-declarators.
(c-font-lock-cut-off-declarators): Check more rigorously that a declaration
being processed starts before the function's starting position.
(c-complex-decl-matchers): Remove the redundant clause which fontified "types
preceded by, e.g., "struct"".

* lisp/progmodes/cc-langs.el (c-template-typename-kwds)
(c-template-typename-key): New lang defconsts and defvar.