[coroutines] Build GRO declaration and return GRO statement

Summary:
1. build declaration of the gro local variable that keeps the result of get_return_object.
2. build return statement returning the gro variable
3. emit them during CodeGen
4. sema and CodeGen tests updated

Reviewers: EricWF, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31646

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@303573 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed