remove dependency on CMake and python under Windows

I'd say we should either do this or add a submodule with pre-build binaries. I think the dowload size is already hugely bloated so I opted into pre-generating project files with CMake and then making the paths relative

If we update the llvm repo all we should need to do is execute UpdateProjectFiles.bat once on a machine that has cmake and python and then push those changes to the main repo.

If there are bigger structural changes it might also be necessary to modify UpdateProjectFiles.bat or make_paths_relative.py 

If all else fails there's still the llvm_build.vcxproj to fall back on if people aren't willing to update the bat and py file.

Anyway, this might as well serve as a place to discuss alternatives (make a binary prebuilds repo, depend on cmake/python like now, make llvm optional, etc.)