[coroutines] Build and pass coroutine_handle to await_suspend

Summary:
This patch adds passing a coroutine_handle object to await_suspend calls.
It builds the coroutine_handle using coroutine_handle<PromiseType>::from_address(__builtin_coro_frame()).

(a revision of https://reviews.llvm.org/D26316 that for some reason refuses to apply via arc patch)

Reviewers: GorNishanov

Subscribers: mehdi_amini, cfe-commits, EricWF

Differential Revision: https://reviews.llvm.org/D30769

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297356 91177308-0d34-0410-b5e6-96231b3b80d8