Synchronous generator support for IIterable<T>

[Fulgen301]

Following up on microsoft/cppwinrt#1323 (comment), I'd like to create a PR that adds it to WIL. However, I am unsure whether this would be the correct project or which file to add it to - coroutine.h does not include winrt.h and winrt.h contains nothing that's coroutine-related.

[dunhor]

cppwinrt_helpers.h already has some coroutine adapters there, so that seems like it's probably the best place (just note to place it within the proper preprocessor guards for detecting coroutine support. coroutine.h is not a C++/WinRT specific header, so that would not be the proper place