Update the autoloads for FILE.
If prefix arg SAVE-AFTER is non-nil, save the buffer too.
If FILE binds `generated-autoload-file' as a file-local variable,
autoloads are written into that file. Otherwise, the autoloads
file is determined by OUTFILE. If called interactively, prompt
for OUTFILE; if called from Lisp with OUTFILE nil, use the
existing value of `generated-autoload-file'.
Return FILE if there was no autoload cookie in it, else nil.
(fn FILE &optional SAVE-AFTER OUTFILE)