[lldb] Capture and load home directory from the reproducer.

When replaying the reproducer, lldb should source the .lldbinit file
that was captured by the reproducer and not the one in the current home
directory. This requires that we store the home directory as part of the
reproducer. By returning the virtual home directory during replay, we
ensure the correct virtual path gets constructed which the VFS can then
find and remap to the correct file in the reproducer root.

This patch adds a new HomeDirectoryProvider, similar to the existing
WorkingDirectoryProvider. As the home directory is not part of the VFS,
it is stored in LLDB's FileSystem instance.