Add missing standard library header inclusions (#5316)

Discovered by clang-tidy.

toolchain/parse/extract.cpp

@@ -2,6 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include <initializer_list>
+#include <optional>
 #include <tuple>
 #include <typeinfo>
 #include <utility>