Revert "Track Clang virtualFileSystem change (r201618)"

This has been reverted upstream as a part of
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201755
91177308-0d34-0410-b5e6-96231b3b80d8 however lldb is lagging
behind slightly.

This reverts commit 0141200d09b9b7a5037e8bac94d3e112eda4691e.