clang-modernize: Transform::createActionFactory return ownership by unique_ptr instead of raw pointer.

Follow up to r213851 to simplify code and reduce the chance of future
leaks.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@215214 91177308-0d34-0410-b5e6-96231b3b80d8