I am in the camp of people that want to move their msvc code to other realms. I am chiming in for political reasons. Our stuff is rudimentary C++ and we use as little of the host os abuse as possible. But you can barely do anything without interfacing to some important libraries.
Dump the hyperbole. It is counter productive.
It seems obvious that the minimum functionality is that you can compile and debug processes with clang that will run on windows. That to me implies being able to _LINK_ with platform libraries. To some extent that means you have to be able to digest their macros. But that is not the same as a wholesale emulation. I see no way around doing at least that much emulation.