[Coroutines] Part14: Handle coroutines with no suspend points.

[Coroutines] Part14: Handle coroutines with no suspend points.

Summary:
If coroutine has no suspend points, remove heap allocation and turn a coroutine into a normal function.

Also, if a pattern is detected that coroutine resumes or destroys itself prior to coro.suspend call, turn the suspend point into a simple jump to resume or cleanup label. This pattern occurs when coroutines are used to propagate errors in functions that return expected<T>.

Reviewers: majnemer

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D24408

llvm-svn: 282414