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

Summary:
Noticed during review of D41102.

I'm not sure whether there are any principal reasons why it returns raw owning pointer,
or it is just a old code that was not updated post-C++11.

I'm not too sure what testing i should do, because `check-all` is not error clean here for some reason,
but it does not //appear// asif those failures are related to these changes.

This is clang part.
Clang-tools-extra part is D43780.

Reviewers: klimek, bkramer, alexfh, pcc

Reviewed By: alexfh

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D43779

llvm-svn: 326201