Make DiagnosticsEngine::takeClient return std::unique_ptr<>

Summary:
Make DiagnosticsEngine::takeClient return std::unique_ptr<>. Updated
callers to store conditional ownership using a pair of pointer and unique_ptr
instead of a pointer + bool. Updated code that temporarily registers clients to
use the non-owning registration (+ removed extra calls to takeClient).

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6294