Rollup merge of #112870 - compiler-errors:clause-2, r=oli-obk

Migrate `item_bounds` to `ty::Clause`

Should be simpler than the next PR that's coming up. Last three commits are the relevant ones.

r? ``@oli-obk`` or ``@lcnr``

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -705,7 +705,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => tcx
                 .explicit_item_bounds(def_id)
                 .subst_iter_copied(tcx, substs)
-                .find_map(find_fn_kind_from_did),
+                .find_map(|(clause, span)| find_fn_kind_from_did((clause.as_predicate(), span))),
             ty::Closure(_, substs) => match substs.as_closure().kind() {
                 ty::ClosureKind::Fn => Some(hir::Mutability::Not),
                 ty::ClosureKind::FnMut => Some(hir::Mutability::Mut),