No Apple is pushing CLANG for exactly the reason that they want to use BSD license in a take not give fashion...how hackable is it; Xcode(SDK) will only work on Mac OS X.
GPL didn't stop Xcode existing when it operated around gcc. Xcode will always be an OS X thing, it has nothing to do with the back end compiler license.
Yes, they get a lot of mileage out of tightly coupling Xcode with llvm - eg: they don't have to write the same level of context sensitive language support for editing when you can do constant incremental compiling and inspect the state of the compiler's trees.
BTW; Apple use LLVM for far more than just Xcode. They used it in the display subsystem to run-time optimize code to the actual machine's display configuration.
Being GPLv3 is a bonus for Apple, but it's about more than that. Competition is a good thing.