Use copy initialization to initialize std::unique_ptr.

Thanks to David Blaikie for the suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215867 91177308-0d34-0410-b5e6-96231b3b80d8
Repace SmallPtrSet with SmallPtrSetImpl in function arguments to avoid needing to mention the size.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215868 91177308-0d34-0410-b5e6-96231b3b80d8