Protect the ArgList dtor

It could even be made non-virtual if it weren't for bad compiler
warnings.

This demonstrates that ArgList objects aren't destroyed polymorphically
and possibly that they aren't even used polymorphically. If that's the
case, it might be possible to refactor the two ArgList types more
separately and simplify the Arg ownership model. *continues
experimenting*

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206727 91177308-0d34-0410-b5e6-96231b3b80d8