Convert CommandLine member references to pointers (#4301)

This follows up on a [style
question](https://discord.com/channels/655572317891461132/821113559755784242/1283516297686286377)
about whether to prefer reference members or pointers. This PR converts
to pointers as a demonstration of that style choice.

Note, I'm trying to update constructors to match use of `*` based on
whether they keep a reference. I'm removing a few `const&` uses where no
reference was kept (i.e., it was just copied, and didn't seem worth a
move).

I'm changing `AddArgImpl` to return an `Arg*` because it just gets
passed to a constructor, seems simpler this way.