Downgrade to LLVM 3.4.2

* 3.5.0 has at least 2 incompatibilities with the generator:
  * llvm::raw_fd_ostream constructor has changed.
    std::ofstream could be used but Clang API expects llvm::raw_ostream
    so much copying is needed (via llvm::raw_string_ostream).
  * different arguments for ASTUnit::LoadFromASTFile.