Fix rust-lang#83943.

Fix handling of `--output-format json` flag

- Don't treat it as deprecated on stable and beta channels. Before, it
  would give confusing and incorrect output:

  ```
  warning: the 'output-format' flag is considered deprecated
    |
    = warning: see issue rust-lang#44136 <rust-lang#44136> for more information

  error: json output format isn't supported for doc generation
  ```
  Both of those are wrong: output-format isn't deprecated, and json
  output is supported.

- Require -Z unstable-options for `--output-format json`

  Previously, it was allowed by default on nightly, which made it hard
  to realize the flag wouldn't be accepted on beta or stable.

To get the test working I had to remove `-Z unstable-options`, which x.py passed to compiletest unconditionally. It was first added in rust-lang@8c2ec68 so `-Z miri` would be allowed. -Z miri is no longer passed unconditionally, so hopefully removing it won't break anything.

r? ```@aDotInTheVoid``` cc ```@HeroicKatora``` ```@CraftSpider```

Thanks to ```@memoryruins``` for pointing it out on Discord!

cc ```@Mark-Simulacrum``` for the change to compiletest.

…dtwco

Add more info for common trait resolution and async/await errors

* Suggest `Pin::new`/`Box::new`/`Arc::new`/`Box::pin` in more cases
* Point at `impl` and type defs introducing requirements on E0277

Account for `ExprKind::Block` when suggesting .into() and deref

Fix rust-lang#83943.

…r=lqd

Add Debug implementation for hir::intravisit::FnKind

Fix outdated crate names in `rustc_interface::callbacks`