c++: refactor some parser code

c++: refactor some parser code

cp_parser_declaration copies tokens to local variables, before inspecting
(some of) their fields.  There's no need.  Just point at them in the token
buffer -- they don't move.  Also, we never look at the second token if the
first is EOF, so no need for some kind of dummy value in that case.

	gcc/cp/
	* parser.c (cp_parser_declaration): Avoid copying tokens.
	(cp_parser_block_declaration): RAII token pointer.