[C++20] [Coroutines] Implement return value optimization for get_return_object

This patch tries to implement RVO for coroutine's return object got from
get_return_object.
From [dcl.fct.def.coroutine]/p7 we could know that the return value of
get_return_object is either a reference or a prvalue. So it makes sense
to do copy elision for the return value. The return object should be
constructed directly into the storage where they would otherwise be
copied/moved to.

Test Plan: folly, check-all

Reviewed By: junparser

Differential revision: https://reviews.llvm.org/D117087