bug#28080: 25.2; `outline-invisible-p' should be specific to `invisible' property of `outline.el'

[Drew Adams]

> > > (It also should not be a `defsubst'.  1985 has come and gone.)
> >
> > Except it is still a defsubst.  It is called within some loops, so
> > perhaps we should at least see some numbers before changing that?
> 
> Doesn't matter what the numbers are, IMHO.  `defsubst' should be
> dropped altogether now.  It was annoying 30 years ago, and it's
> still annoying - for no real benefit. ;-)

(2017 has also come and gone.)  This is still a defsubst.
Could we please get it promoted to a defun now?  Thx.