[coroutines] Implement unhandled_exception changes.

Summary:
This patch adopts the recent changes that renamed `set_exception(exception_pointer)` to `unhandled_exception()`. 

Additionally `unhandled_exception()` is now required, and so an error is emitted when exceptions are enabled but the promise type does not provide the member.
When exceptions are disabled a warning is emitted instead of an error, The warning notes that the `unhandled_exception()` function is required when exceptions are enabled.

Reviewers: rsmith, GorNishanov, aaron.ballman, majnemer

Reviewed By: GorNishanov

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D30859

llvm-svn: 298565