[coroutines] Add co_return statement emission

Summary:
Added co_return statement emission.

Tweaked coro-alloc.cpp test to use co_return to trigger coroutine processing instead of co_await, since this change starts emitting the body of the coroutine and await expression handling has not been upstreamed yet.

Reviewers: rsmith, majnemer, EricWF, aaron.ballman

Reviewed By: rsmith

Subscribers: majnemer, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D29979

llvm-svn: 297076