[lldb] Remove FileSystem::Initialize from VFS mapping

This patch removes the ability to instantiate the LLDB FileSystem class
based on a VFS overlay. This also removes the "hack" where we cast the
VFS to a RedirectingFileSystem to obtain the external path. You can
still instantiate a FileSystem with a VFS, but with the caveat that
operations that rely on the external path won't work.

Differential revision: https://reviews.llvm.org/D120923