[coroutines] Part 4[ab]: Coroutine Devirtualization: Lower coro.resume and coro.destroy.

This is the forth patch in the coroutine series. CoroEaly pass now lowers coro.resume
and coro.destroy intrinsics by replacing them with an indirect call to an address
returned by coro.subfn.addr intrinsic. This is done so that CGPassManager recognizes
devirtualization when CoroElide replaces a call to coro.subfn.addr with an appropriate
function address.

Patch by Gor Nishanov!

Differential Revision: https://reviews.llvm.org/D22998

llvm-svn: 277765