Meson for windows with LLVM

This is now just the remaining work to be able to use LLVM with meson on windows.

Worth noting is that this only enables llvmpipe on appveyor. Debian does not provide an LLVM built for mingw (nor could I find any builds readily), and building LLVM from source is not something to do in the container setup (it would make building a new container take forever). SWR is not built on either, due to the fact that SWR has hardcoded assumptions that GCC == Linux and MSVC == Windows; as well as requiring a newer version of LLVM than we use in the appveyor CI.

I'd really like to get this all done and in for 19.3 if at all possible.