Allow multi-component paths in VFS file nodes

This allows the 'name' field to contain a path, like

{ 'type': 'directory',
  'name': '/path/to/dir',
  'contents': [ ... ] }

which not only simplifies reading and writing these files (for humans),
but makes it possible to easily modify locations via textual
replacement, which would not have worked in the old scheme.

E.g. sed s:<ROOT>:<NEW ROOT>

llvm-svn: 202109