[coroutines] Add support for llvm.coro.noop intrinsics

Summary:
A recent addition to Coroutines TS (https://wg21.link/p0913) adds a pre-defined coroutine noop_coroutine that does nothing.
To implement this feature, we implemented an llvm.coro.noop intrinsic that returns a coroutine handle to a coroutine that does nothing when resumed or destroyed.

Reviewers: EricWF, modocache, rnk, lewissbaker

Reviewed By: modocache

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45114

llvm-svn: 328986