Fix the build with MSVC 2013 by explicitly requesting llvm::make_unique

MSVC 2013 provides std::make_unique, which it finds with ADL when one of
the parameters is std::unique_ptr, leading to an ambiguous overload.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207597 91177308-0d34-0410-b5e6-96231b3b80d8