enable optimizations for coroutines

[andrewrk]

Right now we set optnone noinline on coroutines because of https://bugs.llvm.org/show_bug.cgi?id=36578

We also don't do the coroutine allocation elision IR because of a bug I didn't file yet. (I implemented it and it crashed LLVM).

This issue is to do both of these optimizations.

[andrewrk]

Solved by not using LLVM's coroutines, merged in #3033.