Revert "[Tooling] [0/1] Refactor FrontendActionFactory::create() to return std::unique_ptr<>"

This reverts commit rL326201

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: 326204