[clangd] Unlink VFS working dir from OS working dir.

A lot of our previous FS manipulation was thread-unsafe in practice with
the RealFS implementation.

This switches to a different RealFS mode where path-manipulation is used
to simulate multiple working dirs.
r351050 both added this mode and removed the cache. If we want to
move back to the old implementation we need to put the cache back.

llvm-svn: 351051