Rather than branching on whether the return type is ByMoveWrapper within
ReturnAction itself, hoist the distinction to outside. This allows the main
class template to be modified without worrying about this special case, which
means we can stop using a shared pointer to the value (introduced as a
linked_ptr in commit 3d1c78b2bf to support ByMove) in this commit and simplify
the class template further in a future commit with the eventual aim of directly
supporting move-only result types.
PiperOrigin-RevId: 445938943
Change-Id: I7bc71ea301d5e493ac6ecbe57d62738a48a2721a
5176726123