LLVM developers have been working on merging code from the long-standing open-source "AArch64" back-end into Apple's "ARM64" back-end. When all of that is complete in the days ahead, Apple's ARM64 compiler back-end with all functionality from the AArch64 back-end will be renamed within the open-source LLVM tree to AArch64.
James Molloy of ARM Holdings provided a status update today in regards to the merging of these two 64-bit ARM back-ends. The final switchover is expected sometime next week and overall there's no major regressions known, the performance is close to the same, and porting of other AArch64-related code is nearly complete.
More information on this 64-bit ARM LLVM work can be found via the LLVM developers' mailing list. This new back-end will be part of the LLVM 3.5 release this summer.