feature-gate `unsafe_no_drop_flag`

[pnkfelix]

As part of non-zeroing dynamic drop (RFC 320), we need to feature-gate unsafe_no_drop_flag so that users do not come to rely on its unstable semantics when we release 1.0.

[pnkfelix]

nominating, seems 1.0 beta to me.

[pnkfelix]

(subtask of #5016, but gets higher priority due to (more) semantic visibility)

[SSheldon]

This feature gate doesn't seem to be required as of alpha 2. The following compiles without issue:

#[unsafe_no_drop_flag]
struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Dropping");
    }
}

fn main() { }

[pnkfelix]

it seems like PR #22364 may have accidentally have removed the gating of unsafe_no_drop_flag.

(That's what I deserve for not including a regression test on this.)

I think I know how to fix this. (And I will look into making sure all of the feature-gated things have tests.)