Reland [Driver] Fix -working-directory issues

Don't change the default VFS in Driver, update tests & reland.

This reverts commit 999f8a7416f8edc54ef92e715fd23c532bcc74d4.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374926 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed