Apart from being cleaner this also means that clang-format no longer has
access to the host file system. This isn't necessary because clang-format
never reads includes :)

Includes minor tweaks and bugfixes found in the VFS implementation while
running clang-format tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249385 91177308-0d34-0410-b5e6-96231b3b80d8