Use overlays in non-font-lock buffers.

Several modes like eshell or cider-repl-mode propertize their contents while
inserting them instead of using font-lock.  When highlighting a symbol in such a
buffer using font-lock, the original face properties get removed and one ends up
with all text shown in default face (except for the highlighted symbol, of
course).

So this patch makes highlight-symbol use overlays instead of font-lock in
buffers where font-lock-keywords is nil.