Swift-DocC should display C++ and Objective-C++ as their own source languages

In https://github.com/apple/swift-docc/pull/766, i'm adding C++ and Objective-C++ as allowed source languages to display in Swift-DocC. However, these are being sorted in with Objective-C, like plain C is currently being handled (#169). This is a stopgap measure; in the future we should investigate what it would take to separate these source languages into their own categories.