[C++ PATCH] Remove K&R declaration hack.

[C++ PATCH] Remove K&R declaration hack.

https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01885.html
	gcc/
	* doc/extend.texi (Backwards Compatibility): Remove implicit
	extern C leeway of () being (...).
	gcc/cp/
	* decl.c (decls_match): Remove SYSTEM_IMPLICIT_EXTERN_C matching
	of return types and parms.
	* parser.c (cp_parser_parameter_declaration_clause): Likewise,
	'()' always means '(void)'.

From-SVN: r264013