This commit landing in mainline Mesa a few minutes ago just adds the new Polaris10 and Polaris11 processor models to the LLVM processor targets with the same feature-set being exposed as current Volcanic Islands graphics processors. The commit itself is not too exciting but AMD's LLVM back-end is an important piece of their modern open-source driver stack.
The Git code is currently tracking for LLVM 3.9 while this small patch will presumably end up in an LLVM 3.8.1 point release too.
Meanwhile, for the rest of the stack, it's looking like the Polaris support will merge for Linux 4.7 (not the current Linux 4.6 cycle) -- assuming the DAL code is merged. The Polaris support for Mesa is currently sitting on the mailing list for the RadeonSI Gallium3D driver. That work should land with plenty of time before the Mesa 11.3/12.0 release in three months time.