Adopt ref in Core.Iterate.Next (#7381)

Change the `cursor` parameter in `Core.Iterate.Next` to be a `ref`
parameter
instead of a pointer parameter.

Assisted-by: Gemini via Antigravity

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/check/handle_loop_statement.cpp

@@ -222,20 +222,15 @@ auto HandleParseNode(Context& context, Parse::ForHeaderId node_id) -> bool {
   auto loop_header_id = StartLoopHeader(context, start_node_id);
   auto continue_depth = context.scope_stack().ambient_cleanup_scope_depth();
 
-  // Call `<range>.(Iterate.Next)(&cursor)`.
-  auto cursor_type_inst_id = context.types().GetTypeInstId(cursor_type_id);
-  auto cursor_addr_id = AddInst<SemIR::AddrOf>(
-      context, node_id,
-      {.type_id = GetPointerType(context, cursor_type_inst_id),
-       .lvalue_id = cursor_var_id});
-  // A range that implements neither fails both lookups; reporting the second
-  // would say the same thing about the same expression a second time.
+  // Call `<range>.(Iterate.Next)(ref cursor)`.
+  // Don't diagnose if the call to `Iterate.NewCursor` failed, since this call
+  // will likely also fail for the same reason.
   // TODO: We should only perform the impl lookup once.
   auto element_id =
       BuildBinaryOperator(context, node_id,
                           {.interface_name = CoreIdentifier::Iterate,
                            .op_name = CoreIdentifier::Next},
-                          range_id, cursor_addr_id,
+                          range_id, cursor_var_id,
                           /*diagnose=*/cursor_id != SemIR::ErrorInst::InstId);
   // We need to convert away from an initializing expression in order to call
   // `HasValue` and then separately pattern-match against the element.