So... it's not apparently possible without modifying the source code. So like I said originally, what Xcode is using for static analysis is apparently Apple's choice, not yours nor mine.
Yes; what's the point? Nobody's making you use Xcode (I don't), and nobody's stopping you using the Clang/LLVM features like static analysis outside of Xcode. If you want, you're free to call clang's static analysis from GNUstep's Xcode clone¹ (assuming GNUstep's license allows that; LLVM's certainly does), or any other IDE or editor.
¹https://github.com/gnustep/gnustep-xcode