After today's iPhone 5S unveiling, the matter of A7 support for LLVM/Clang was brought up on the mailing list. Apple is a key backer to LLVM/Clang and employ many of its key contributors. LLVM/Clang is used now at the heart of both OS X and iOS and has superseded GCC in Apple's development stack. While Apple is surely using LLVM/Clang on the A7 SoC, they aren't yet ready to talk about it or commit open-source support.
In response to the mailing list thread, Eli Friedman a key Clang contributor and Apple employee, simply wrote, "We can't say anything at the moment."
The A7-based iPhone 5S will be released before month's end so hopefully at that time we'll have a better idea how it compares to ARM's AArch64 and in due time see open-source support in upstream LLVM/Clang -- hopefully before the 3.4 release before year's end.
Update: LLVM project leader Chris Lattner has now responded with, "A number of you have asked about the 64-bit CPU in the iPhone 5s, and what that means for LLVM. The iPhone 5s is based on the ARMv8 / Aarch64 instruction set, but the clang compiler in Xcode 5 is based on a custom LLVM Aarch64 backend, not the one currently on llvm.org. Apple is committed to contributing its Aarch64 backend to the community (merging it "the right way" with the existing backend), but it was a significant amount of work, and will take at least several months to work out all the details. I'll keep you posted."