Remove some transient raw pointer ownership in ClangAttrEmitter::createArgument

This function might be a bit easier if it were split in two with a lot
of early returns - and that setOptional bit in the outer function, but
anyway.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215263 91177308-0d34-0410-b5e6-96231b3b80d8