[VFS] Don't run symlink test on Windows, it may pass or fail

This test was XFAILed because of symlinks, but some versions of ln -s
seem to work on Windows, so the test was unexpectedly passing on our
bot:
http://lab.llvm.org:8011/builders/clang-x64-windows-msvc/builds/13233
Unexpected Passing Tests (1):
    Clang :: VFS/subframework-symlink.m

I don't know how or why, but it seems dependent on system configuration,
and is not something worth debugging. Avoid the problem by marking the
test UNSUPPORTED: system-windows instead of XFAIL: system-windows.