Reapply [VFS] Allow multiple RealFileSystem instances with independent CWDs.

This reverts commit r351091.
The original mac breakages are addressed by ensuring the root directory
we're working from is fully symlink-resolved before starting.

Differential Revision: https://reviews.llvm.org/D58169

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354026 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed