An interesting question about slant range.
Having said all that, I don't know the answer to your question as to whether any radar computer tries to correct for slant angle, but I suspect not. In the en-route environment the error is infinitesimal...
In an en-route environment, as a target goes overhead the radar at, say 30,000 ft, the error would be something approaching 6 miles.
Most SSR display systems can correct for this error. Those which combine primary and secondary sources or multiple sources do some very clever mathematics before ploting the target on the screen. The downside of all of this clever stuff, along with things like track prediction, is that the target displayed to a controller is not necessarily where the aeroplane is!
P.S. - not trying to scaremonger, when determined correctly radar separation standards take such things into account.