Fix more VFS tests on Windows

Since VFS paths can be in either Posix or Windows style, we have to use
a more flexible definition of "absolute" path.

The key here is that FileSystem::makeAbsolute is now virtual, and the
RedirectingFileSystem override checks for either concept of absolute
before trying to make the path absolute by combining it with the current
directory.

Differential Revision: https://reviews.llvm.org/D70701