Revert "[Tooling] [1/1] Refactor FrontendActionFactory::create() to return std::unique_ptr<>"

This reverts commit rL326202

This broke gcc4.8 builds, compiler just segfaults:
http://lab.llvm.org:8011/builders/clang-atom-d525-fedora-rel/builds/14909
http://lab.llvm.org:8011/builders/clang-x86_64-linux-abi-test/builds/22673

llvm-svn: 326203