a modern Objective-C Ecosystem, comparable to iOS and XCode from Apple, the well known market leader, blah blah
making it a perfect porting target for iOS applications to run on any cheap laptop with any Linux distribution
Correctly implemented library compatible-with-UIKit is a massive undertaking. Switching compilers won’t fix this or speed it up greatly.
It might attract new people who would more easily take a look at it, but, honestly, given the amount of effort, MRC is comparably not going to be the problem here.
Core Animation-compatible implementation needs more work, as does Opal (Core Graphics). And if you just want to do what was planned years ago — adapt Chameleon to GNUstep’s AppKit — you still need to finish the CA-AppKit integration work.
These things are not enabled by switching to Clang, they are possible with GCC with similar amount of effort as working in a possible Clang fork. Only one thing comes to mind that is enabled by Clang+libobjc2: @dynamic properties in Core Animation (but, only if you care about @dynamic animatable properties being done well).
So let’s not drag iOS into the story. ARC, ObjC++ and other language features are what’s addressable here, and whether core GS should allow this.