Add missing standard library header inclusions (#5316)

Discovered by clang-tidy.

toolchain/check/handle_function.cpp

@@ -2,6 +2,9 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include <optional>
+#include <utility>
+
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/context.h"
 #include "toolchain/check/control_flow.h"