Add "enum classs" support to C++ mode.

Add "enum classs" support to C++ mode.
progmodes/cc-langs.el (c-after-brace-list-decl-kwds)
(c-after-brace-list-key): New language consts/variables.
progmodes/cc-engine.el (c-looking-at-decl-block): Exclude
spurious match of "enum struct" from decl-block recognition.
(c-backward-colon-prefixed-type): New function.
(c-backward-over-enum-header): Call above function to extend
recognition of enum structure.