Parent-Task:

• SR-15577 AutoDiff ABI stability

Issue Description:

`@callee_guaranteed` closure convention is not suitable for AD pullback closures because, combined with its `@owned` parameter convention for the closure context argument, it will always incur an extra retain. In most use cases pullbacks are called exactly once, and the context argument will be consumed directly by the entry basic block. We should change linear maps’ callee convention to `@owned`.

I had a WIP PR earlier if anyone wants to build on that. #34935