Depends on https://reviews.llvm.org/D71900.
The fourth in a series of patches that ports the LLVM coroutines passes
to the new pass manager infrastructure. This patch implements
No existing regression tests check the behavior of coro-cleanup on its
own, so this patch adds one. (A test named 'coro-cleanup.ll' exists, but
it relies on the entire coroutines pipeline being run. It's updated to
test the new pass manager in the 5th patch of this series.)