Honestly, Clang does seem like it would be pretty easy to embed in comparison. They do mention their app is written in C#, and integrating C++ code with it on every platform may carry its own challenges. I know it's relatively easy to do with desktop platforms with Mono and Microsoft .NET just using normal PInvoke, but have no idea about iOS; code signing looks complicated on iOS, though I'm sure you know it inside and out. Either way... yeah, if you consider Clang, it really does appear like a waste of time to write your own C++ compiler, even for just a subset.