I have the same git-svn config and am able to
git svn fetch successfully, so it might be something specific to your setup.
A potential alternative would be using the monorepo (http://llvm.org/docs/GettingStarted.html#for-developers-to-work-with-a-git-monorepo), although you probably want to use the “official” monorepo at https://github.com/llvm-git-prototype/llvm instead (it should be moved over to the LLVM organization on GitHub in a few days, at which point it’ll officially be official, but I think the actual repository itself will remain unchanged at this point). The long-term plan is to move all LLVM development to the monorepo, so it might be best to just transition over your workflow right now? It’ll still go through svn for committing, but TortoiseSVN has worked well for me on Windows in the past.