Stop skewing inference in ?'s desugaring

[WaffleLapkin]

NB: this is a breaking change (although arguably a bug fix) and as such shouldn't be taken lightly.

This changes expr?'s desugaring like so (simplified, see code for more info):

// old
match expr {
    Ok(val) => val,
    Err(err) => return Err(err),
}

// new
match expr {
    Ok(val) => val,
    Err(err) => core::convert::absurd(return Err(err)),
}

// core::convert
pub const fn absurd<T>(x: !) -> T { x }

This prevents ! from the return from skewing inference:

// previously: ok (never type spontaneous decay skews inference, `T = ()`)
// with this pr: can't infer the type for `T`
Err(())?;

Fixes #51125
Closes #39216