[coroutines] [NFC] Add tests for return_void, unhandled_exception and promise dtor

Summary:
* Test that coroutine promise destructor is called.
* Test that we call return_void on fallthrough
* Test that we call unhandled exception in a try catch surrounding the body

Reviewers: EricWF, GorNishanov

Reviewed By: GorNishanov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33479

llvm-svn: 303748