Upgrade gen and bootstrap to use Clang 3.9

[zimmski]

We are currently using Clang 3.4 which makes using new features of Clang, i.e., the address sanitizer, impossible. It would make it also easier to integrate changes for new Clang versions since switching to the newer version repositories is not needed any more.

This would not conflict with generating updates to the version repositories since the would be generated using their Clang version.

[sbinet]

SGTM
(I must say though, that even my archlinux hasn't seen clang-3.9 percolating through all the pipes :P)

[zimmski]

openSUSE's latest is 3.8.1, so I am in the same situation as you. But llvm.org has packages for Debian/Ubuntu and they are already white-flagged for TravisCI and that is basically our development environment. Still a lot to do until I get there though.