b) Take over -fabi-version=3 and bump the template arg change to v4.
I'm not sure what "take over" means, but I think this change should be
in v4. People might be using v3, and we shouldn't break their world.
Introducing 2.1, or a separate flag, seems odd to me; there's nothing
particularly more or less different about this change then other
mangling changes, as far as I can tell. It's another case where we've
discovered an inability to implement the full language with the current
scheme, and have therefore been forced to make a change.