rustdoc elides `TyParam: Sized` bounds on inlined cross-crate re-exported associated items even if they're meaningful

[theemathas]

The documentation for add in std is missing a where T: Sized clause.

Note, however, that the corresponding documentation in core correctly has the where T: Sized clause.

@rustbot labels +T-rustdoc

[fmease]

Affects all predicates of the form TyParam: Sized on IXCRE'd associated items where TyParam comes from a parent item and is ?Sized.

We mindlessly filter out IXCRE TyParam: Sized from the list of lowered(!) predicates to reconstruct pre-Sized-elabored clauses (i.e., ɛ → ?Sized, Sized → ɛ) but we don't take into account that such a Sized bound has to be user-written if the param comes from the parent item.

[fmease]

MCVE (doesn't matter if the container is a trait, trait impl or inherent impl):

pub trait Tr<T: ?Sized> { fn f() where T: Sized; }