Create the non-numeric backup file name for FILE.
Normally this will just be the file's name with `~' appended.
Customization hooks are provided as follows.
If the variable `make-backup-file-name-function' is non-nil, its value
should be a function which will be called with FILE as its argument;
the resulting name is used.
Otherwise a match for FILE is sought in `backup-directory-alist'; see
the documentation of that variable. If the directory for the backup doesn't exist, it is created.