[clang] Return std::unique_ptr<TargetInfo> from AllocateTarget

In file 'clang/lib/Basic/Targets.cpp' the function 'AllocateTarget' had a raw pointer as a return type, which have been wrapped in the 'std::unique_ptr' in all usages.
This commit changes the signature of the function to return an instance of 'std::unique_ptr' directly.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D148574