Coroutine variant fields can be uninitialized

Wrap coroutine variant fields in MaybeUninit to indicate that they
might be uninitialized. Otherwise an uninhabited field will make
the entire variant uninhabited and introduce undefined behaviour.

The analogous issue in the prefix of coroutine layout was addressed by
6fae7f8071.

compiler/rustc_ty_utils/src/layout.rs

@@ -831,7 +831,10 @@ fn coroutine_layout<'tcx>(
                     Assigned(_) => bug!("assignment does not match variant"),
                     Ineligible(_) => false,
                 })
-                .map(|local| subst_field(info.field_tys[*local].ty));
+                .map(|local| {
+                    let field_ty = subst_field(info.field_tys[*local].ty);
+                    Ty::new_maybe_uninit(tcx, field_ty)
+                });
 
             let mut variant = univariant_uninterned(
                 cx,