c++: use G_ instead of _

Since these strings are passed to error_at, they should be marked for
translation with G_, like other diagnostic messages, rather than _, which
forces immediate (redundant) translation.  The use of N_ is less
problematic, but also imprecise.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_primary_expression): Use G_.
	(cp_parser_using_enum): Likewise.
	* decl.cc (identify_goto): Likewise.