added llvm 3.4

I added formula for LLVM 3.4.
LLVM 3.4 is now in-progress and I couldn't find snapshot archives like
gcc weekly snapshots. So I decided to use the latest source code from
repositories.

This formula is now head-only and should be updated when official LLVM
3.4 is released.

make llvm34 head only

because llvm 3.4 sources are provided in only SVN repositories

fix for installing clang

use man1 helper.