[C++11] Using std::unique_ptr to ensure that Argument objects do not leak (since clang-tblgen isn't long-lived, the old leak is probably acceptable, but it offended my senses nonetheless).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202989 91177308-0d34-0410-b5e6-96231b3b80d8