FindLLVM.cmake now starts searching for specific "llvm-config-x.y"
versions instead of using the system-wide default "llvm-config" first.
This avoids breaking builds on Debian 8, where LLVM 3.5 is the (yet
unsupported) default, but 3.4 is still installable.
Change-Id: I6fd577f515a233e30c6f803f87b9a680b5515a5b