PR c/23143
	* c-parser.c (c_parser_parms_list_declarator): Call
	mark_forward_parm_decls.
	* c-decl.c (merge_decls): Only check DECL_IN_SYSTEM_HEADER for
	decls with visibility structure.

testsuite:
	* gcc.dg/parm-forwdecl-1.c, gcc.dg/parm-forwdecl-2.c,
	gcc.dg/parm-forwdecl-3.c, gcc.dg/parm-forwdecl-4.c: New tests.

From-SVN: r102581