AnyInvocable is a C++11 compatible equivalent of the C++23 [std::move_only_function](https://en.cppreference.com/w/cpp/utility/functional/move_only_function/move_only_function).
Although this implementation matches an intermediate draft revision of the standard (http://wg21.link/p0288r5), it is neither a standard tracking type nor a seamless backfill type.

PiperOrigin-RevId: 455494585
Change-Id: If01565f8eecc78eee38fb794ef142b32b31abc7c