move `EarlyBinder` into `ImplTraitHeader`

[lcnr]

tcx.impl_trait_header currently returns EarlyBinder<ImplTraitHeader>. Move this EarlyBinder into the ImplTraitHeader to only wrap the TraitRef:

pub struct ImplTraitHeader<'tcx> {
    pub trait_ref: EarlyBinder<TraitRef<'tcx>>,
    pub polarity: ImplPolarity,
    pub unsafety: Unsafety,
}

[Y-Nak]

@rustbot claim