Also fix some formatting along the way.

Use Applicability::Unspecified for all of them instead.

This turns an ICE on this code:

fn main() {
    |_: [u8; break]| ();
}

from
    'assertion failed: self.tcx.sess.err_count() > 0', librustc_typeck/check/mod.rs
to
    librustc_mir/hair/cx/expr.rs:543: invalid loop id for break: not inside loop scope

which is at a later stage during compilation and most importantly
fixes of bug #50576 will fix this as well.

Fixes #50802

No error checking or feature gating yet

It's redundant as Result already implements Encodable
as well as Decodable.

When we want to implement label-break-value,
we can't really decide whether to emit ScopeTarget::Loop or
ScopeTarget::Block in the code that is supposed to create it.
So we get rid of it and reconstruct the information when
needed.

Like #43008 (f6689991), but _much more aggressive_.