Reverting the virtual file system implementation, because it triggers an assertion
in our internal build bots.

This reverts commits 201618, 201635, 201636, 201639, 201685, 201691, and 201696.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201755 91177308-0d34-0410-b5e6-96231b3b80d8