[VFS] Transition clang-format to use an in-memory FS.

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: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@249385 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
1 file changed