[coroutines] Implement correct GRO lifetime

Summary:
Sema creates a declaration for gro variable as:

auto $gro = $promise.get_return_object();

However, gro variable has to outlive coroutine frame and coroutine promise, but,
it can only be initialized after the coroutine promise was created, thus, we
split its emission in two parts: EmitGroAlloca emits an alloca and sets up
the cleanups. Later when the coroutine promise is available we initialize
the gro and set the flag that the cleanup is now active.

Duplicate of: https://reviews.llvm.org/D31670 (which arc patch refuses to apply for some reason)

Reviewers: GorNishanov, rsmith

Reviewed By: GorNishanov

Subscribers: EricWF, cfe-commits

Differential Revision: https://reviews.llvm.org/D33477

llvm-svn: 303716