x.py clippy argument passing broken, order no loger preserved

[matthiaskrgr]

https://github.com/rust-lang/rust/blob/master/src/bootstrap/src/core/build_steps/check.rs#L66

apparently we now group the args passed to x.py clippy by lint level (allow, deny, etc) and then send group by group to the clippy command.

This is bad because you can no longer turn on/off single lints, for example x.py clippy -Aclippy:all -Wclippy::style -Aclippy::foo1 -Aclippy::foo2 used to only turn on style lints with the exception of foo1 and foo2.

The order is no longer preserved

        args.extend(strings(&["--", "--cap-lints", "warn"]));
        args.extend(ignored_lints.iter().map(|lint| format!("-Aclippy::{}", lint)));
        let mut clippy_lint_levels: Vec<String> = Vec::new();
        allow.iter().for_each(|v| clippy_lint_levels.push(format!("-A{}", v)));
        deny.iter().for_each(|v| clippy_lint_levels.push(format!("-D{}", v)));
        warn.iter().for_each(|v| clippy_lint_levels.push(format!("-W{}", v)));
        forbid.iter().for_each(|v| clippy_lint_levels.push(format!("-F{}", v)));
        args.extend(clippy_lint_levels);
        args.extend(builder.config.free_args.clone());

which would now become -Aclippy::all -Aclippy::foo1 -Aclippy::foo2 -Wclippy::style which kind of defeats the purpose of being able to setting specific lints to allow/warn.