Another important side benefit of switching from GCC to Clang/LLVM is the freedom from aspects of the GPL that aren't compatible with the way Apple operates. Also, the license Clang/LLVM is under just "jives" better with the BSD foundation upon which OSX and iOS are both based, IMHOIt has everything to do with this thread topic.
LLVM is essential to Apple being able to straddle ARM and Intel architectures without too much developer strife.
https://twitter.com/d0k/status/228224463710326784 (Mountain Lion first OS release compiled with CLANG/LLVM)
Due to the speculation that performance gaps between Intel and ARM chips will not be closing any time soon, I think Apple is serious about this, but they'll spring it on us in a way we haven't thought of. Here's an example: Imagine something resembling the 2012 iMacs in physical dimensions, but with a unique Apple twist: The main system board is Intel-based just like Macs of the past. What makes this one different is the 48- to 64-core ARM "daughtercard", based on a derivative of the A-series chips powering Apple's iOS family, installed in one of the expansion slots. Just imagine the possibilities this would open up - you get the best of the Intel and ARM worlds in one device.