[Coroutines] Fix premature conversion of return object

Fix https://github.com/llvm/llvm-project/issues/56532

Effectively, this reverts behavior introduced in https://reviews.llvm.org/D117087,
which did two things:

1. Change delayed to early conversion of return object.
2. Introduced RVO possibilities because of early conversion.

This patches fixes (1) and removes (2). I already worked on a follow up for (2)
in a separated patch. I believe it's important to split these two because if the RVO
causes any problems we can explore reverting (2) while maintaining (1).

Notes on some testcase changes:
- `pr59221.cpp` changed to `-O1` so we can check that the front-end honors
  the value checked for. Sounds like `-O3` without RVO is more likely
  to work with LLVM optimizations...
- Comment out delete members `coroutine-no-move-ctor.cpp` since behavior
  now requires copies again.

Differential Revision: https://reviews.llvm.org/D145639