Update LLVM version: 3.5 => 3.6

We branched 3.5, it's now time to work on 3.6.

This is Sylvestre's patch from [1] plus regenerated configure
file by me, and minus the release notes reset, which Sean
pointed out [2] should happen later.

 1. http://reviews.llvm.org/D4660
 2. http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20140721/111137.html