Use unique_ptr to handle ownership of synthesized args in DerivedArgList

This might be able to be simplified further by using Arg as a value type
in a linked list (to maintain pointer validity), but here's something
simple to start with.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206724 91177308-0d34-0410-b5e6-96231b3b80d8