When I ran apt to install a package not listed in the Software Center, I got a warning message that libllvm4.0 was still present on my system but not needed, and advising me to use autoremove on the command line to remove it. I tried to find information on this package, and all I got was that it's some kind of C compiler library. I have g++ and various tools installed, but I'm not sure whether that installattion added this library to my system or it's left over from the (recent) operating system install or subsequent Software Updates.
I'm running Ubuntu 16.04.3, installed from an official .iso file I downloaded last night and burned to a USB drive using Rufus from within a Win 10 OS. Rufus downloaded some isolinux-related files to complete the burning of the iso to the USB drive, but I don't know whether that inadvertently added this library to my install. I no longer have access to the Rufus downloads folder as I overwrote Windows during the install.