Pretty-print generators with their `generator_kind`

Pretty-print generators with their `generator_kind`

After removing `GenFuture`, I special-cased async generators to pretty-print as `impl Future<Output = X>` mainly to avoid too much diagnostics changes originally.

This now reverses that change so that async fn/blocks are pretty-printed as `[$movability `async` $something@$source-position]` in various diagnostics, and updates the tests that this touches.