Bump known-good LLVM revision

Author: eliben

README.rst

@@ -24,7 +24,7 @@ Last known LLVM build revision for the master branch
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The last upstream LLVM revision I've successfully built the ``master`` branch
-against is r308224 (17-Jul-2017). It may build with newer revisions, or it may
+against is r314221 (26-Sep-2017). It may build with newer revisions, or it may
 not. If you know it builds successfully with a newer one, please let me know and
 I'll update this note.