Auto merge of #22026 - kmcallister:plugin, r=sfackler

Auto merge of #22026 - kmcallister:plugin, r=sfackler

```rust
#[plugin] #[no_link] extern crate bleh;
```

becomes a crate attribute

```rust
#![plugin(bleh)]
```

The feature gate is still required.

It's almost never correct to link a plugin into the resulting library / executable, because it will bring all of libsyntax and librustc with it. However if you really want this behavior, you can get it with a separate `extern crate` item in addition to the `plugin` attribute.

Fixes #21043.
Fixes #20769.

[breaking-change]