Rollup merge of #122843 - WaffleLapkin:semicolon-vs-the-never, r=compiler-errors

Add a never type option to make diverging blocks `()`

More experiments for ~~the blood god~~ T-lang!

Usage example:
```rust
 #![allow(internal_features)]
 #![feature(never_type, rustc_attrs)]
 #![rustc_never_type_options(diverging_block_default = "unit")]

fn main() {
    let _: u8 = { //~ error: expected `u8`, found `()`
        return;
    };
}
```

r? compiler-errors

I'm not sure how I feel about parsing the attribute every time we create `FnCtxt`. There must be a better way to do this, right?

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