[CN] Improve error message for bad ordering of ghost arguments in function specifications

[dimecon]

For example, currently

int foo(int x)
/*@
  requires
    x + n - n < MAXi32();
    cn_ghost i32 n; true;
  ensures
    return == x + 1i32; 
@*/
{
    return x + 1;
}

gives

return code: 1
tests/cn/ghost_bad_ordering.error.c:5:5: warning: experimental keyword 'cn_ghost' (use of experimental features is discouraged)
    cn_ghost i32 n; true;
    ^~~~~~~~ 
tests/cn/ghost_bad_ordering.error.c:5:5: error: unexpected token after ';' and before 'cn_ghost'
Please add error message for state 1696 to parsers/c/c_parser_error.messages
    cn_ghost i32 n; true;
    ^~~~~~~~ 

rems-project/cn#170 (comment)