Apple is quite clear what are the official languages for iOS apps and what role C++ plays on that, like MSL and DriverKit. Note how many C++ talks happen at WWDC.
You're pretty much stuck with clang, unless you feel like hacking around the toolchain workflows.
These days I still use macOS and exist in the Apple ecosystem. I wouldn't mind moving to Linux, but unfortunately I need a MacBook to do work where I build iOS apps. I wonder if there is a way around that.